Don't disable extensions immediately if verification is out of date

If the install verification signature we have stored in our prefs is
older than the install time of an extension, we don't want to
immediately disable it. Instead, we want to do another server request to
update our signature, and then disable if appropriate.

BUG=342165
R=finnur@chromium.org

Review URL: https://codereview.chromium.org/160313002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250855 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250855