Part 3 of immutable Extension refactor.
Make ExtensionsService hold const Extension pointers only. This ensures that
extensions can't be modified after they're created, and lets us share them
between threads.
BUG=56558
TEST=no functional change
Review URL: http://codereview.chromium.org/4138006
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@64517 0039d316-1c4b-4281-b951-d872f2087c98