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This is a combined revert of
https://codereview.chromium.org/1491653002
and https://codereview.chromium.org/1614173002. One more
additional edit is the addition of
"testRunner.dumpAsText();" in canvas-toBlob-defaultpng.html
and canvas-toBlob-toDataURL-race.js.

Reason for this revert is that we found out idle-task
performs much better (has less negative impacts on other
activities) than threaded implementation on majority of
machines.

BUG=581574

Review URL: https://codereview.chromium.org/1637163002

Cr-Commit-Position: refs/heads/master@{#371806}