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When the ExecutionContext is destroyed, the outstanding request is marked as cancelled.

The corresponding chromium side of this change is https://codereview.chromium.org/341923004/

BUG=375430

Review URL: https://codereview.chromium.org/343723003

git-svn-id: svn://svn.chromium.org/blink/trunk@176502 bbb929c8-8fbe-4397-9dbb-9b2b20218538