Added 'if ENABLE_EXTENSIONS' guards around the Activity Log invocations

The Activity Log only exists if extensions are enabled. This makes sure that callers never try to fetch it unless extensions are enabled.

BUG=
R=kalman@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23440053

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224673 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224673