Remove InstallTracker's extension load, unload observer.
Since ExtensionRegistry provide canonical way, use  ExtensionRegistry instead.

BUG=374353
TEST=unit_tests

Review URL: https://codereview.chromium.org/298023009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275434 0039d316-1c4b-4281-b951-d872f2087c98
15 files changed