Show the extension install UI when the user clicks Reenable for a disabled-on-upgrade extension.

Also fix a couple misc bugs with disabled extensions:
- Fix a crash when trying to reenable an uninstalled disabled extension.
- Fix a bug where upgrading a disabled extension would reenable it.

BUG=12140
TEST=no

Review URL: http://codereview.chromium.org/172006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@23675 0039d316-1c4b-4281-b951-d872f2087c98
12 files changed