Bug 1134888 - Disable the mediasource-config-change-webm-a-bitrate.html test; r=cpearce

It is unreliableon Windows.

testing/web-platform/meta/media-source/mediasource-config-change-webm-a-bitrate.html.ini

@@ -0,0 +1,3 @@
+[mediasource-config-change-webm-a-bitrate.html]
+  type: testharness
+  disabled: https://bugzilla.mozilla.org/show_bug.cgi?id=1134888