Fully instrument canvas2d context operations.

As part of this, don't digest context operations if currently in a
RequestAnimationFrame context.

For new canvas2d APIs that aren't enabled yet, set the "some operations
were skipped during digesting" taint bit and add a TODO to consider
instrumenting these in a future CL.

Bug: 973801
Change-Id: Iab9cfae660db557b8db1b2c8171f4bd02bfdb441
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611465


Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Asanka Herath <asanka@chromium.org>
Reviewed-by: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908714}