Fixed Extension change-handling methods to avoid unrecoverable errors/crashes on adding new extensions with extensions sync turned on.

BUG=46742
TEST=manual, trybots

Review URL: http://codereview.chromium.org/2811011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@50068 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed