DOMUI: Fix storing checkbox prefs.
DOMUI: Fix storing checkbox prefs.

The old code only ran value type initialization on DOMContentLoaded,
which is now never hit because the Pref* classes are now created on
DOMContenteLoaded.

BUG=66045
TEST=none

Review URL: http://codereview.chromium.org/5688003

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@68780 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser