Add NULL check to AppRestoreService

Turns out extension hosts can have their extensions become NULL in some cases, e.g. the extension is unloaded.

TBR=koz@chromium.org

BUG=155871

Review URL: https://codereview.chromium.org/11275031

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@164035 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed