MediaPlayer::getSupportedTypes is used by MIMETypeRegistry::initializeSupportedMediaMIMETypes to build a static list of all media MIME types supported by a port. MediaPlayer::getSupportedTypes takes a HashSet<String>&, which it passes to ever registered media engine's getSupportedTypes factory method, which of course means that each call overwrites the values added by the previous engine.
This works as long as every registered media engine supports the same set of MIME types, or the last engine registered is a superset of them all. r191721 added a MediaStream engine to the Mac port. This engine happens to get registered last, so MediaPlayer::getSupportedTypes returns an empty hash set.
Created attachment 264313 [details]
Attachment 264313 [details] did not pass style-queue:
ERROR: Source/WebCore/ChangeLog:8: You should remove the 'No new tests' and either add and list tests, or explain why no new tests were possible. [changelog/nonewtests] 
Total errors found: 1 in 2 files
If any of these errors are false positives, please file a bug against check-webkit-style.
Created attachment 264316 [details]
Comment on attachment 264316 [details]
Clearing flags on attachment: 264316
Committed r191734: <http://trac.webkit.org/changeset/191734>
All reviewed patches have been landed. Closing bug.