GitLab

- Make InstallCrx, PackAndInstallCrx return the installed Extension*.
- Change StartCrxInstall callers to use InstallCrx.
- Fix dubious use of ScopedTempDir in PackAndInstallCrx.
- Add an InstallState enum for the above functions.
- Remove all uses of extensions()->at(), since ExtensionSet won't support it.

BUG=104091
TEST=unit_tests


Review URL: http://codereview.chromium.org/8598028

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@111213 0039d316-1c4b-4281-b951-d872f2087c98