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These IDs introduce variance in digest values for the same set of canvas
2D context operations.

Also, add some unit test coverage that we were missing.

Bug: 973801
Change-Id: I12728c5ab4dcd46b6a21bbcfbb18066652ebbf93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2243731
Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792371}