Fix scrolling to highlighted extension.

This should fix the following URLs that scroll to or highlight extensions:
chrome://extensions/?id=<extension_id>
chrome://extensions/#<extension_id>

BUG=305800
R=arv@chromium.org
TEST=right click an extension -> "Manage" -> should be scrolled into view
NOTRY=true

Review URL: https://codereview.chromium.org/26767002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227942 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227942