Fix crash in Extension::GetBrowserImages()

BUG=150763
TEST=successfully installed browser action extension without default icon defined in manifest

Review URL: https://codereview.chromium.org/10945026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@157560 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed