Remove PrefObserver usage, batch 3.

This also introduces an accessor for the PrefServiceBase* kept by
PrefChangeRegistrar.

TBR=bauerb@chromium.org,achuith@chromium.org
BUG=155525

Review URL: https://chromiumcodereview.appspot.com/11293249

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@167680 0039d316-1c4b-4281-b951-d872f2087c98
13 files changed