Open and close Views Comboboxes on F4, not Space.

Show the Combobox menu on F4 without modifiers (not Space).
Add MenuController::accept_on_f4_ flag and setter.
(optionally handle to accept the pending item, like Return)
Add MenuRunner::COMBOBOX type for MenuController init.
Specify MenuRunner::COMBOBOX in Combobox::ShowDropDownMenu.

TODO(msw): Add tests in followup: http://crrev.com/26567014
(menu tests seem to work but are currently disabled on Win)

BUG=293293
TEST=Combobox menus open and close (accepting the selection) on F4.
R=sky@chromium.org

Review URL: https://codereview.chromium.org/26535006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228570 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228570