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This is the most Chrome-dependent part of //apps, and is only used for the app
list in Chrome, so why not move it to chrome/browser/ui/app_list?

This enables us to make the rest of the //apps code independent of Chrome so it
can be used for app_shell.

Alternatively, we could make custom_launcher_page_contents.cc not depend on
Chrome (it's mostly for extensions::ChromeExtensionWebContentsObserver), but
that's probably better off being lumped into a larger task to move app_list out
of chrome if that should become a goal.

BUG=679971
R=benwells@chromium.org

Review-Url: https://codereview.chromium.org/2628883002
Cr-Commit-Position: refs/heads/master@{#443286}