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We are implementing an ipc layer for GPU command buffer, so the command
buffer should not know the underneath ipc (see CL). To achive this goal,
we need modify mojo command buffer interface to match the current chrome
gpu ipc.

CL https://codereview.chromium.org/1656433002/

BUG=None

Review URL: https://codereview.chromium.org/1686543004

Cr-Commit-Position: refs/heads/master@{#374714}