Allow chunk cancelation only if nothing has been consumed from it.

The mediaChunks.size() > 1 check was supposed to ensure this, and
did roughly the right thing when there was only a single stream
(although it was unnecessarily restrictive in preventing chunk
cancelation for the first chunk, where bytesLoaded != 0 and none
of the samples had been consumed).

Now we have multiple streams the check doesn't do the right thing,
and adding a back-buffer feature will make even more incorrect.
This change switches to checking the condition we actually want
to check directly :).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=174449398

library/core/src/main/java/com/google/android/exoplayer2/source/chunk/ChunkSampleStream.java

@@ -315,7 +315,7 @@ public class ChunkSampleStream<T extends ChunkSource> implements SampleStream, S
       IOException error) {
     long bytesLoaded = loadable.bytesLoaded();
     boolean isMediaChunk = isMediaChunk(loadable);
-    boolean cancelable = !isMediaChunk || bytesLoaded == 0 || mediaChunks.size() > 1;
+    boolean cancelable = bytesLoaded == 0 || !isMediaChunk || !haveReadFromLastMediaChunk();
     boolean canceled = false;
     if (chunkSource.onChunkLoadError(loadable, cancelable, error)) {
       canceled = true;
@@ -415,6 +415,22 @@ public class ChunkSampleStream<T extends ChunkSource> implements SampleStream, S
     return chunk instanceof BaseMediaChunk;
   }
 
+  /**
+   * Returns whether samples have been read from {@code mediaChunks.getLast()}.
+   */
+  private boolean haveReadFromLastMediaChunk() {
+    BaseMediaChunk lastChunk = mediaChunks.getLast();
+    if (primarySampleQueue.getReadIndex() > lastChunk.getFirstSampleIndex(0)) {
+      return true;
+    }
+    for (int i = 0; i < embeddedSampleQueues.length; i++) {
+      if (embeddedSampleQueues[i].getReadIndex() > lastChunk.getFirstSampleIndex(i + 1)) {
+        return true;
+      }
+    }
+    return false;
+  }
+
   /* package */ boolean isPendingReset() {
     return pendingResetPositionUs != C.TIME_UNSET;
   }