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This is part of a larger refactoring. The overall call is to have two Stub
implementations in the GPU process:
- a RasterInterface client will have a RasterCmdBuffStub on the server side and
- a GLES2Interface client will have a GLES2CmdBuffStub on the server side

Rationale for having different stubs are that they are coupled to the decoder,
context, and scheduling. In the future, the RasterInterface backend will diverge
from the GLES2Interface backed in these respects.

Step 1: Create CommandBufferStub class.
Step 2 (separate CL): Have GLES2CmdBuffStub and RasterCmdBuffStub inherit from
CommandBufferStub (see crrev.com/c/796951 for WIP patch).

BUG=789238

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ic15f80bed7fc0e7e4637832378e6502a77568c73
Reviewed-on: https://chromium-review.googlesource.com/806719
Commit-Queue: Jonathan Backer <backer@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521683}