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: http://src.chromium.org/svn/trunk/src@68780 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: eeb15bf1f8a059465ada391da07d2b457a1c3e4d
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