Pass FrameSinkId by const ref

RenderWidgetHostViewBase::GetFrameSinkId passed the viz::FrameSinkId
by value and was not a const method.

This CL returns a const ref of FrameSinkId and makes GetFrameSinkId a
const method so it can be accessed even if the caller has a const
pointer or reference to the RenderWidgetHostViewBase object.

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I5142a75f21a1c0396ba868bf66699a2fb85a2c33
Bug: 672962
Reviewed-on: https://chromium-review.googlesource.com/1141985
Commit-Queue: Fady Samuel <fsamuel@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576275}