Fixes a typo: use the default image if the extension doesn't have icon.

BUG=343115
R=dewittj@chromium.org, atwilson@chromium.org

Review URL: https://codereview.chromium.org/160923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251715 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251715