[Mojo Video Capture] Simplify media::VideoCaptureDevice::Client:Buffer to a struct

media/capture/video/video_capture_buffer_pool_impl.cc

Index: media/capture/video/video_capture_buffer_pool_impl.cc
diff --git a/media/capture/video/video_capture_buffer_pool_impl.cc b/media/capture/video/video_capture_buffer_pool_impl.cc
index 7bc10e16e9e2b4bcd2f18b186e64c5d3ad3cfc48..6c1c43809783ddad6080bc7b1ffd1aa02fb3d158 100644
--- a/media/capture/video/video_capture_buffer_pool_impl.cc
+++ b/media/capture/video/video_capture_buffer_pool_impl.cc
@@ -40,17 +40,16 @@ mojo::ScopedSharedBufferHandle VideoCaptureBufferPoolImpl::GetHandleForTransit(
 }
 
 std::unique_ptr<VideoCaptureBufferHandle>
-VideoCaptureBufferPoolImpl::GetBufferHandle(int buffer_id) {
+VideoCaptureBufferPoolImpl::GetReadWriteAccess(int buffer_id) {
   base::AutoLock lock(lock_);
 
   VideoCaptureBufferTracker* tracker = GetTracker(buffer_id);
   if (!tracker) {
     NOTREACHED() << "Invalid buffer_id.";
-    return std::unique_ptr<VideoCaptureBufferHandle>();
+    return nullptr;
   }
 
-  DCHECK(tracker->held_by_producer());
-  return tracker->GetBufferHandle();
+  return tracker->GetReadWriteAccess();
 }
 
 int VideoCaptureBufferPoolImpl::ReserveForProducer(