Limit extent of webstore app to just chrome.google.com/webstore.

      BUG=93497
TEST=Try installing extensions and apps from the webstore, starting both being
initially logged in, and not.


Review URL: http://codereview.chromium.org/7719003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@97986 0039d316-1c4b-4281-b951-d872f2087c98