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This will let Omaha know the difference between new, on-demand installs
that are manually initiated from the webstore, and the background
autoupdate checks that we do for already installed extensions/apps.

BUG=249187

Review URL: https://chromiumcodereview.appspot.com/17524010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@208048 0039d316-1c4b-4281-b951-d872f2087c98