Bug 1126052 - Reenable test_SeekTwice.mp4. r=RyanVM

dom/media/mediasource/test/mochitest.ini

@@ -35,8 +35,7 @@ skip-if = (toolkit == 'android' || buildapp == 'mulet') #timeout android/mulet o
 [test_SeekableBeforeEndOfStream.html]
 [test_SeekableBeforeEndOfStreamSplit.html]
 [test_SeekTwice_mp4.html]
-#skip-if = os == 'linux' # No fmp4 support on linux
-skip-if = true # Fails on most platforms and was pushed anyway.
+skip-if = ((os == "win" && os_version == "5.1") || (os != "win" && os != "mac")) # Only supported on osx and vista+
 [test_SetModeThrows.html]
 [test_SplitAppendDelay.html]
 [test_SplitAppend.html]