Centralize FrameSinkId allocation in one place

content/browser/compositor/gpu_process_transport_factory.cc

Index: content/browser/compositor/gpu_process_transport_factory.cc
diff --git a/content/browser/compositor/gpu_process_transport_factory.cc b/content/browser/compositor/gpu_process_transport_factory.cc
index 713812a1012a9e7a75bde89fbb3c362a4d9426a0..fbdf14b4a687acf59025cc0de3a08d078dd27400 100644
--- a/content/browser/compositor/gpu_process_transport_factory.cc
+++ b/content/browser/compositor/gpu_process_transport_factory.cc
@@ -689,12 +689,7 @@ GpuProcessTransportFactory::GetContextFactoryPrivate() {
 }
 
 cc::FrameSinkId GpuProcessTransportFactory::AllocateFrameSinkId() {
-  // The FrameSinkId generated here must be unique with

[danakj, 2017/02/15 20:18:10]

This comment goes away, but we lose any notion of the importance that this
doesn't collide with RenderWidgetHostImpl, which seems bad. It's also not
tracable in the code anymore.

How can we make the code actually enforce this instead, or connect it back to a
place that understands it at least, like we do here?

[xlai (Olivia), 2017/02/15 22:38:36]

Done. I added the comment back in places where FrameSinkIdAllocator is
constructed.

-  // RenderWidgetHostViewAura's
-  // and RenderWidgetHostViewMac's FrameSinkId allocation.
-  // TODO(crbug.com/685777): Centralize allocation in one place for easier
-  // maintenance.
-  return cc::FrameSinkId(0, next_sink_id_++);
+  return frame_sink_id_allocator_.NextFrameSinkId();
 }
 
 void GpuProcessTransportFactory::SetDisplayVisible(ui::Compositor* compositor,