Correct comment in extension_function_test_utils.cc for better understanding.

BUG=None

Review URL: https://codereview.chromium.org/23876033

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226043 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226043