Fix typeahead for popup menu with attributes

Popup menu item with attributes (<optgroup> for instance) will
break typeahead matching support on Mac. Use the same trick Apple
uses in WebKit2 [1] to fix this.

BUG=90064

[1] http://trac.webkit.org/browser/trunk/Source/WebKit2/UIProcess/mac/WebPopupMenuProxyMac.mm#L90

Review URL: https://codereview.chromium.org/99043003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238377 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238377