Re-add an early exit to GetOrCreateExtensionList. The safeguard is useful during testing but I had optimistically removed it because browser_tests were passing locally. However, the bots didn't like it.

TBR=kalman
BUG=328441

Review URL: https://codereview.chromium.org/136793006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244674 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244674