Tweak IsExtensionEnabled and consolidate callers of related functions

BUG=none
TEST=ExtensionService*, ExtensionPrefs*

Review URL: http://codereview.chromium.org/7330029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@92047 0039d316-1c4b-4281-b951-d872f2087c98
6 files changed