Fixes and improvements to cloud policy for extensions.

- wait for the ExtensionService to be ready before processing install
  notifications.

- avoid creating the managed storage leveldb cache if there are no policies set
  for that extension.

- filter the initial policies loaded from the component policy cache.

- always publish policy at the ForwardingPolicyProvider, even if nothing new
  was added; this sends the right notification later if a component is added
  back.

- don't trigger updates when a SchemaRegistry gets an update with an empty
  map.

- check if the Schema for an extension being shown in about:policy is valid
  before reading it.

BUG=108992

Review URL: https://codereview.chromium.org/78953002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237111 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237111