Fix build when build flag enable_webrtc is disabled.
This fixes a build regression introduced by https://codereview.chromium.org/72603004/ where the EnumerationFailed callback was removed.

TEST= change common.gypi 'enable_webrtc%': 0, and compile.

Review URL: https://codereview.chromium.org/102393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238593 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238593