Prevent frozen frames when the decoder is always late.

Create a MediaCodecVideoTrackRenderer.shouldDropFrame function that can be overriden by a child class. The YouTube override prevents a frame drop if we haven't rendered anything in the last 35 ms.

The YouTube override is off at the moment, I plan to use a server side flag to do a slow and controlled experiment.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=142190774

library/src/main/java/com/google/android/exoplayer2/video/MediaCodecVideoRenderer.java

@@ -403,7 +403,7 @@ public class MediaCodecVideoRenderer extends MediaCodecRenderer {
         bufferPresentationTimeUs, unadjustedFrameReleaseTimeNs);
     earlyUs = (adjustedReleaseTimeNs - systemTimeNs) / 1000;
 
-    if (earlyUs < -30000) {
+    if (shouldDropOutputBuffer(earlyUs, elapsedRealtimeUs)) {
       // We're more than 30ms late rendering the frame.
       dropOutputBuffer(codec, bufferIndex);
       return true;
@@ -437,6 +437,17 @@ public class MediaCodecVideoRenderer extends MediaCodecRenderer {
     return false;
   }
 
+  /**
+   * Returns true if the current frame should be dropped.
+   *
+   * @param earlyUs Time indicating how early the frame is. Negative values indicate late frame.
+   * @param elapsedRealtimeUs Wall clock time.
+   */
+  protected boolean shouldDropOutputBuffer(long earlyUs, long elapsedRealtimeUs) {
+    // Drop the frame if we're more than 30ms late rendering the frame.
+    return earlyUs < -30000;
+  }
+
   private void skipOutputBuffer(MediaCodec codec, int bufferIndex) {
     TraceUtil.beginSection("skipVideoBuffer");
     codec.releaseOutputBuffer(bufferIndex, false);