GpuMemoryBufferVideoFramePool: add upload TRACE

GpuMemoryBufferVideoFramePool::CopyVideoFrameToGpuMemoryBuffers schedules
copies to GMBs using a Pool thread, so the upload is sliced and run in
parallel, reconvening after all sub tasks are finished.

This CL adds an ASYNC TRACE to track the duration of the full copy cycle,
encompassing all the parallel (and possibly heterogenous) uploads, to
make it easy to reason about how much time the copy-to-GMBs take.

Bug: 803975
Change-Id: Ied827b30b43931ca06996c97dd8f855b75c9ff7a
Reviewed-on: https://chromium-review.googlesource.com/886962Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532101}

media/video/gpu_memory_buffer_video_frame_pool.cc

@@ -610,6 +610,9 @@ void GpuMemoryBufferVideoFramePool::PoolImpl::OnCopiesDone(
     }
   }
 
+  TRACE_EVENT_ASYNC_END0("media", "CopyVideoFrameToGpuMemoryBuffers",
+                         video_frame->timestamp().InNanoseconds() /* id */);
+
   media_task_runner_->PostTask(
       FROM_HERE,
       base::Bind(&PoolImpl::BindAndCreateMailboxesHardwareFrameResources, this,
@@ -655,6 +658,8 @@ void GpuMemoryBufferVideoFramePool::PoolImpl::CopyVideoFrameToGpuMemoryBuffers(
     }
   }
 
+  TRACE_EVENT_ASYNC_BEGIN0("media", "CopyVideoFrameToGpuMemoryBuffers",
+                           video_frame->timestamp().InNanoseconds() /* id */);
   // Post all the async tasks.
   for (size_t i = 0; i < num_planes; i += planes_per_copy) {
     gfx::GpuMemoryBuffer* buffer =