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Before this, a task is added to the idle queue each time in flush and
ScheduleMoreIdleWork. This means that as long as there is idle work, the
size of the queue grows by one in each flush.

Fix this by not adding any idle tasks if there is already one pending.

BUG=401712

Review URL: https://codereview.chromium.org/450793002

Cr-Commit-Position: refs/heads/master@{#288283}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288283 0039d316-1c4b-4281-b951-d872f2087c98