[Mac] Paste in Omnibox needs to initiate field editing correctly.

We override -paste: to do Chrome-specific stuff.  This circumvents
certain Cocoa-standard setup which happens when user's edit text
views, and elsewhere we depend on that setup being right.  This
changes our code to participate in that setup process.

http://crbug.com/21301
TEST=Copy "raising arizona" to the clipboard.  Put focus in the NTP
(Omnibox field does not have focus ring).  Click in Omnibox field and
paste.  Should now see autocomplete popup.

Review URL: http://codereview.chromium.org/213029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26620 0039d316-1c4b-4281-b951-d872f2087c98