Extract an ExtensionRegistry from ExtensionService

This new class holds the various sets of enabled/disabled/terminated/etc.
extensions. It lives in src/extensions/browser so it can be used by
app_shell.

This CL renames some members and methods to clarify which set of extensions is the enabled set and also renames some methods that are only used in test code.

BUG=none
TEST=unit_tests ExtensionRegistryTest and ExtensionServiceTest

Review URL: https://codereview.chromium.org/111873008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242248 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242248