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Functionality:

The |AppRegistry| has for each running application an |AppActivityRegistry|.

The |AppActivityRegistry| knows all activities associated with the application it represents.
It can furthermore shut the app entirely down upon resource manager request. It will then create
an |AppActivityProxy| for the overview mode which shows a placeholder for an unloaded app. This
placeholder can then ask the |AppActivityRegistry| to restart the application again.

A shutdown request for the application is only performed when all activities were marked for
UNLOAD.

If there were multiple activities upon shutdown for one app, the app has to take care of
re-creating all windows and thus re-creating all activities. Since an activity match cannot
be performed, the |AppActivityProxy| will only be shown once and it will show in the location
of the most recently used activity of that app. If we later on find an app which really uses
multiple windows and it is imperative to keep the history for all of them tact & the app is
recreating them properly, (a lot of if's) we can revisit the single |AppActivityProxy| and
try to address it in a cleaner way, but at this time that seems rather un-useful since it is
not known if required.

BUG=388085
TEST=AppActivityTest.*

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=291221

Review URL: https://codereview.chromium.org/477523002

Cr-Commit-Position: refs/heads/master@{#291454}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291454 0039d316-1c4b-4281-b951-d872f2087c98