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Due to loadTimeData inconsistencies between refreshes, it's possible for
"chromeCleanupEnabled" to become true while no ChromeCleanupHandler is
present. 
To avoid showing a blank card or crashing in debug mode for edge cases,
always install the handler.

Also added some documentation to load_time_data.js to hopefully help
other developers avoid this issue.

Bug: 690020
Change-Id: Ifa8e0c0e78c2d6202e11fd378c74781b24401918
Reviewed-on: https://chromium-review.googlesource.com/565327Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: proberge <proberge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485391}