Add buffer length metric to ABR playback test.

This metric is measured once every second.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161527345

testutils/src/main/java/com/google/android/exoplayer2/testutil/ActionSchedule.java

@@ -17,6 +17,7 @@ package com.google.android.exoplayer2.testutil;
 
 import android.os.Handler;
 import android.view.Surface;
+import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.SimpleExoPlayer;
 import com.google.android.exoplayer2.testutil.Action.ClearVideoSurface;
 import com.google.android.exoplayer2.testutil.Action.Seek;
@@ -91,11 +92,18 @@ public final class ActionSchedule {
      * @return The builder, for convenience.
      */
     public Builder apply(Action action) {
-      ActionNode next = new ActionNode(action, currentDelayMs);
-      previousNode.setNext(next);
-      previousNode = next;
-      currentDelayMs = 0;
-      return this;
+      return appendActionNode(new ActionNode(action, currentDelayMs));
+    }
+
+    /**
+     * Schedules an action to be executed repeatedly.
+     *
+     * @param action The action to schedule.
+     * @param intervalMs The interval between each repetition in milliseconds.
+     * @return The builder, for convenience.
+     */
+    public Builder repeat(Action action, long intervalMs) {
+      return appendActionNode(new ActionNode(action, currentDelayMs, intervalMs));
     }
 
     /**
@@ -175,6 +183,13 @@ public final class ActionSchedule {
       return new ActionSchedule(rootNode);
     }
 
+    private Builder appendActionNode(ActionNode actionNode) {
+      previousNode.setNext(actionNode);
+      previousNode = actionNode;
+      currentDelayMs = 0;
+      return this;
+    }
+
   }
 
   /**
@@ -184,6 +199,7 @@ public final class ActionSchedule {
 
     private final Action action;
     private final long delayMs;
+    private final long repeatIntervalMs;
 
     private ActionNode next;
 
@@ -197,8 +213,19 @@ public final class ActionSchedule {
      * @param delayMs The delay between the node being scheduled and the action being executed.
      */
     public ActionNode(Action action, long delayMs) {
+      this(action, delayMs, C.TIME_UNSET);
+    }
+
+    /**
+     * @param action The wrapped action.
+     * @param delayMs The delay between the node being scheduled and the action being executed.
+     * @param repeatIntervalMs The interval between one execution and the next repetition. If set to
+     *     {@link C#TIME_UNSET}, the action is executed once only.
+     */
+    public ActionNode(Action action, long delayMs, long repeatIntervalMs) {
       this.action = action;
       this.delayMs = delayMs;
+      this.repeatIntervalMs = repeatIntervalMs;
     }
 
     /**
@@ -234,6 +261,9 @@ public final class ActionSchedule {
       if (next != null) {
         next.schedule(player, trackSelector, surface, mainHandler);
       }
+      if (repeatIntervalMs != C.TIME_UNSET) {
+        mainHandler.postDelayed(this, repeatIntervalMs);
+      }
     }
 
   }