[MediaRecorder] Don't hop to the main thread to retrieve frames.

This gives VideoTrackRecorderImpl a WebGraphicsContext3DProvider
bound to the encoder thread so frames can be retrieved inline
before encoding instead of having to hop to the main thread.

Local testing shows this is ~10% faster in terms of unit test run time;
~16s to ~14s.

To facilitate this change, the existing worker-thread based context
creation used by WebGLRenderingContextBase is extracted into a
utility class in "blink/renderer/platform/graphics/".

Fixed: 1138712
Change-Id: I137f4f6830ac1c7b0860f6c65584a85b276607a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495867
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820870}