Make Mac clipboard code tolerant of nil clipboard without crashing.  Add a DCHECK to catch this case, and clean up some of the clipboard code. Review URL: http://codereview.chromium.org/6580
Make Mac clipboard code tolerant of nil clipboard without crashing.  Add a
DCHECK to catch this case, and clean up some of the clipboard code.
Review URL: http://codereview.chromium.org/6580

git-svn-id: http://src.chromium.org/svn/trunk/src@3051 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 4d7d5d7b11e9a7f937ecc8e4d9117fed6c135095
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser