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Previously, if a user navigated to a subpage directly via URL, i.e.
chrome://settings/searchEngines, and then tried to close that subpage,
the animation would not work correctly.

This patch fixes that issue.

BUG=535013

Review URL: https://codereview.chromium.org/1372713002

Cr-Commit-Position: refs/heads/master@{#351189}