Simplify initialization of WebGL context limits.

WebGLRenderingContextBase now queries them directly from the
WebGraphics3DContextProvider rather than delegating this
responsibility to the DrawingBuffer, which might fail to allocate.

Bug: 933528
Change-Id: I9164a491436b32bf255afcf242c814743f3eb69e
Reviewed-on: https://chromium-review.googlesource.com/c/1478394
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633556}
4 files changed