Remove WebGraphicsContext3D reason from the CauseForGpuLaunch enum

Replace "WEBGRAPHICSCONTEXT3DCOMMANDBUFFERIMPL_INITIALIZE" with a bunch
of more specific and real reasons.

R=piman
BUG=584497

Review-Url: https://codereview.chromium.org/1950723002
Cr-Commit-Position: refs/heads/master@{#391721}
8 files changed