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For ensuring that the blink is always initialized with WebThread, the
call sites of blink must implement WebThread to expose the task runner.
UtilityThread is the only case where blink is run without exposing the
platform thread. This CL adds new WebThread implementation for
UtilityThread. After this change blink will always initialized with
platform thread and task runner exposed.

BUG=486782

Review URL: https://codereview.chromium.org/1133563007

Cr-Commit-Position: refs/heads/master@{#329442}