Clear page display after navigation if no rendered output arrives

A bug reporter found a way to navigate a page and cause the omnibar to
update with the destination's URL, but block the renderer from producing
any compositor frames using script. This has some implications for
phishing, even though the phisher's actual content has been unloaded
from the renderer process.

This CL creates a timeout which clears the last displayed compositor
frame if no update is received within 2 seconds after the navigation
being committed.

BUG=497588

Review URL: https://codereview.chromium.org/1339803002

Cr-Commit-Position: refs/heads/master@{#348902}

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