Make extensions keyword providers accept empty input.
Make extensions keyword providers accept empty input.

BUG=63505
TEST=n/a

Review URL: http://codereview.chromium.org/5203001

git-svn-id: http://src.chromium.org/svn/trunk/src@66539 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 58fbe2f3619660d8ba2dab3a550229350215c933
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