Bug 1128115: Update web-ref tests. r=karlt

Generalize [Test removesourcebuffer event on activeSourceBuffers.] expected result to all platforms using mp4

testing/web-platform/meta/media-source/mediasource-removesourcebuffer.html.ini

@@ -2,6 +2,6 @@
   type: testharness
   [Test removesourcebuffer event on activeSourceBuffers.]
     expected:
-      if not debug and (os == "win") and (version == "6.2.9200") and (processor == "x86_64") and (bits == 64): FAIL
-      if not debug and (os == "win") and (version == "6.1.7601") and (processor == "x86") and (bits == 32): FAIL
+      if (os == "win") and (version != "5.1.2600"): FAIL
+      if os == "mac": FAIL