Bug 1267887 - Document MP4AudioInfo::IsValid(). r=jya

Jean-Yves and I both found this logic confusing.

MozReview-Commit-ID: C5phemRPDQZ

media/libstagefright/binding/DecoderData.cpp

@@ -156,6 +156,7 @@ bool
 MP4AudioInfo::IsValid() const
 {
   return mChannels > 0 && mRate > 0 &&
+         // Accept any mime type here, but if it's aac, validate the profile.
          (!mMimeType.Equals(MEDIA_MIMETYPE_AUDIO_AAC) ||
           mProfile > 0 || mExtendedProfile > 0);
 }