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1. Remove the OnClientConnectionClosed() IPC from GPU to browser.  The
   browser never used the IPC so it wasn't necessary.
2. Destroy CompositorFrameSinkImpl when the client connection is closed.
   The object is useless after that point so it can be cleaned up.
3. Don't destroy RootCompositorFrameSinkImpl when the client connection
   is closed. The display compositor will continue to produce frames.

Bug: 814475
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: Ie9a96ee504013435c23b05cf6dbaa3fda4e0fd33
Reviewed-on: https://chromium-review.googlesource.com/993993Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Saman Sami <samans@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548305}