[rAC, OSX] Default text for combo boxes.

All combo boxes for requestAutocomplete have a concept of a default
value. Support that and ensure that is the initial selection.

BUG=277314
R=rsesek@chromium.org

Review URL: https://codereview.chromium.org/25145002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226100 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226100