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They are only created in ResourceProviderTest, not used in production.
Also remove ScopedWriteLockGpuMemoryBuffer and helper functions in
LayerTreeResourceProvider.

Bug: None
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I8ddd10fe5c0f86f994dbfeb966200154304d8432
Reviewed-on: https://chromium-review.googlesource.com/1011558
Commit-Queue: Antoine Labour <piman@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551087}