Hide WebGraphicsContext3DCommandBufferImpl.

This makes the WebGraphicsContext3DCommandBufferImpl inside the
ContextProviderCommandBuffer an implementation detail of the class,
removing it from the constructor. Now callers pass all the interesting
data directly to the ContextProviderCommandBuffer constructor and it
stores them as members, passing them to InitializeOnCurrentThread in
WebGraphicsContext3DCommandBufferImpl when setting up the context.

R=piman@chromium.org, sievers
BUG=584497

Review-Url: https://codereview.chromium.org/1937313003
Cr-Commit-Position: refs/heads/master@{#391166}
14 files changed