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We make the ContextProviderCommandBuffer and its WebGraphicsContext3D
private in the command buffer version of the tests, and use a
GLInProcessContext in the in-process version. The shared code only is
exposed to GLES2Interface and ContextSupport.

R=kbr@chromium.org
BUG=584497

Review URL: https://codereview.chromium.org/1807813002

Cr-Commit-Position: refs/heads/master@{#381598}