Fix the announcement of combo boxes by excluding them from firing events on our combo...

Fix the announcement of combo boxes by excluding them from firing events on our combobox IAccessible.  Since we already call SetFocus on the native combo box, this event will go to the AT correctly, but this IAccessible won't be hooked up to the rest of the IAccessible tree.  Also, any changes made in combobox.cc/h concerning accessibility won't show up when the combobox receives focus.
BUG=48703
TEST=none
Review URL: http://codereview.chromium.org/2937004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@52132 0039d316-1c4b-4281-b951-d872f2087c98