Add MobileFirstEditInOmnibox UMA event.

BUG=178932

Review URL: https://codereview.chromium.org/104873014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241818 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241818