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If there is an untriggered idle task when the document is gone, it
introduces memory leak because IdleRequestCallbackWrapper keeps a
reference to the ScriptedIdleTaskController, making it live longer
than the document.

This patch fixes the issue by letting IdleRequestCallbackWrapper
store a WeakPersistent instead.

BUG=595155

Review-Url: https://codereview.chromium.org/2618893002
Cr-Commit-Position: refs/heads/master@{#441929}