Fix 4729. Make StyleInjector.flush public.

Patch by: stephen.haberman
Review at http://gwt-code-reviews.appspot.com/1614806

Review by: rchandia@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11032 8db76d5a-ed1c-0410-87a9-c151d255dfc7