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Now that all blink threads have schedulers it makes sense to get
hold the corresponding WebScheduler via WebThread.  This patch
is the chromium side of doing that.

Patch 2: https://codereview.chromium.org/1087203002
Patch 3: https://codereview.chromium.org/1084173002

BUG=463143

Review URL: https://codereview.chromium.org/1088053003

Cr-Commit-Position: refs/heads/master@{#325450}