[media] Clarify PipelineImpl locking comments.

Bug: 893739
Change-Id: Icb19a36549d9f877f5b32a5a10b05f1420f812a8
Reviewed-on: https://chromium-review.googlesource.com/c/1279292
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Reviewed-by: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599882}

media/base/pipeline_impl.cc

@@ -80,9 +80,10 @@ class PipelineImpl::RendererWrapper : public DemuxerHost,
       base::OnceClosure change_completed_cb);
 
  private:
-  // Contains state shared between main and media thread.
-  // Main thread can only read. Media thread can both - read and write.
-  // So it is not necessary to lock when reading from the media thread.
+  // Contains state shared between main and media thread. On the media thread
+  // each member can be read without locking, but writing requires locking. On
+  // the main thread reading requires a lock and writing is prohibited.
+  //
   // This struct should only contain state that is not immediately needed by
   // PipelineClient and can be cached on the media thread until queried.
   // Alternatively we could cache it on the main thread by posting the
@@ -93,6 +94,10 @@ class PipelineImpl::RendererWrapper : public DemuxerHost,
   struct SharedState {
     // TODO(scherkus): Enforce that Renderer is only called on a single thread,
     // even for accessing media time http://crbug.com/370634
+    //
+    // Note: Renderer implementations must support GetMediaTime() being called
+    // on both the main and media threads. RendererWrapper::GetMediaTime() calls
+    // it from the main thread (locked).
     std::unique_ptr<Renderer> renderer;
 
     // True when OnBufferedTimeRangesChanged() has been called more recently
@@ -536,6 +541,7 @@ void PipelineImpl::RendererWrapper::OnEnded() {
 
 // TODO(crbug/817089): Combine this functionality into renderer->GetMediaTime().
 base::TimeDelta PipelineImpl::RendererWrapper::GetCurrentTimestamp() {
+  DCHECK(media_task_runner_->BelongsToCurrentThread());
   DCHECK(demuxer_);
   DCHECK(shared_state_.renderer || state_ != kPlaying);
 

media/base/renderer.h

@@ -56,6 +56,8 @@ class MEDIA_EXPORT Renderer {
   virtual void SetVolume(float volume) = 0;
 
   // Returns the current media time.
+  //
+  // This method must be safe to call from any thread.
   virtual base::TimeDelta GetMediaTime() = 0;
 
   // Provides a list of DemuxerStreams correlating to the tracks which should