Ship canvas.toBlob

With 3 L-g-t-m from the API owners
(https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/FnZBCN4t46w),
this feature is ready to ship.

Patches that make corrections based on specification change
 (https://codereview.chromium.org/1496683005 and
https://codereview.chromium.org/1609313002/) have been landed. Also, the
concern of having the threaded implementation (as compared to the idle-periods
implementation) have been addressed and fixed by using a shared thread pool in
Blink (https://code.google.com/p/chromium/issues/detail?id=567887).

BUG=67587

Review URL: https://codereview.chromium.org/1617063003

Cr-Commit-Position: refs/heads/master@{#371256}
3 files changed