The default action (browse the webstore) in the Extension Action context menu
should, as with other platforms, be disabled on OS X when there's no valid
webstore link.

BUG=331614
R=sail@chromium.org

Review URL: https://codereview.chromium.org/144293003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246961 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246961