Fix Widevine L3 provisioning in V2

1. HttpMediaDrmCallback.executeProvisionRequest needs to specify
   an empty byte[], else we do a GET instead of a POST.
2. Content-Type should not be set when making the provision
   request, since there's no body.
3. DataSource implementations must correctly handle a non-null
   body with zero length. CronetDataSource was not handling this
   case. DefaultHttpDataSource was, but made a code modification
   to make it a little clearer. OkHttpDataSource seems to handle
   the case correctly, and it doens't look like the code can be
   made clearer.

Issue #1925

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136042641

extensions/cronet/src/main/java/com/google/android/exoplayer2/ext/cronet/CronetDataSource.java

@@ -416,8 +416,10 @@ public class CronetDataSource extends UrlRequest.Callback implements HttpDataSou
         executor, cronetEngine);
     // Set the headers.
     synchronized (requestProperties) {
-      if (dataSpec.postBody != null && !requestProperties.containsKey(CONTENT_TYPE)) {
+      if (dataSpec.postBody != null && dataSpec.postBody.length != 0
-        throw new OpenException("POST request must set Content-Type", dataSpec, Status.IDLE);
+          && !requestProperties.containsKey(CONTENT_TYPE)) {
+        throw new OpenException("POST request with non-empty body must set Content-Type", dataSpec,
+            Status.IDLE);
       }
       for (Entry<String, String> headerEntry : requestProperties.entrySet()) {
         requestBuilder.addHeader(headerEntry.getKey(), headerEntry.getValue());
@@ -434,11 +436,14 @@ public class CronetDataSource extends UrlRequest.Callback implements HttpDataSou
       }
       requestBuilder.addHeader("Range", rangeValue.toString());
     }
-    // Set the body.
+    // Set the method and (if non-empty) the body.
     if (dataSpec.postBody != null) {
+      requestBuilder.setHttpMethod("POST");
+      if (dataSpec.postBody.length != 0) {
         requestBuilder.setUploadDataProvider(new ByteArrayUploadDataProvider(dataSpec.postBody),
             executor);
       }
+    }
     return requestBuilder.build();
   }
 

library/src/main/java/com/google/android/exoplayer2/drm/HttpMediaDrmCallback.java

@@ -71,7 +71,7 @@ public final class HttpMediaDrmCallback implements MediaDrmCallback {
   @Override
   public byte[] executeProvisionRequest(UUID uuid, ProvisionRequest request) throws IOException {
     String url = request.getDefaultUrl() + "&signedRequest=" + new String(request.getData());
-    return executePost(url, null, null);
+    return executePost(url, new byte[0], null);
   }
 
   @Override
@@ -81,6 +81,7 @@ public final class HttpMediaDrmCallback implements MediaDrmCallback {
       url = defaultUrl;
     }
     Map<String, String> requestProperties = new HashMap<>();
+    requestProperties.put("Content-Type", "application/octet-stream");
     if (C.PLAYREADY_UUID.equals(uuid)) {
       requestProperties.putAll(PLAYREADY_KEY_REQUEST_PROPERTIES);
     }
@@ -93,8 +94,6 @@ public final class HttpMediaDrmCallback implements MediaDrmCallback {
   private byte[] executePost(String url, byte[] data, Map<String, String> requestProperties)
       throws IOException {
     HttpDataSource dataSource = dataSourceFactory.createDataSource();
-    // Note: This will be overridden by a Content-Type in requestProperties, if one is set.
-    dataSource.setRequestProperty("Content-Type", "application/octet-stream");
     if (requestProperties != null) {
       for (Map.Entry<String, String> requestProperty : requestProperties.entrySet()) {
         dataSource.setRequestProperty(requestProperty.getKey(), requestProperty.getValue());

library/src/main/java/com/google/android/exoplayer2/upstream/DefaultHttpDataSource.java

@@ -413,11 +413,16 @@ public class DefaultHttpDataSource implements HttpDataSource {
     connection.setInstanceFollowRedirects(followRedirects);
     connection.setDoOutput(postBody != null);
     if (postBody != null) {
+      connection.setRequestMethod("POST");
+      if (postBody.length == 0) {
+        connection.connect();
+      } else  {
         connection.setFixedLengthStreamingMode(postBody.length);
         connection.connect();
         OutputStream os = connection.getOutputStream();
         os.write(postBody);
         os.close();
+      }
     } else {
       connection.connect();
     }