Add an ExtensionPrefStore, layered between the user prefs and the managed prefs, to manage preferences set by extensions.
Update various callers of the PrefValueStore constructor accordingly.

The initial user will be the proxy extension API.

BUG=266
TEST=covered by unit tests
Review URL: http://codereview.chromium.org/2823037

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@52088 0039d316-1c4b-4281-b951-d872f2087c98
23 files changed