Bug 1143971: Part2. Re-enable mediasource-duration.html tests. r=karlt a=testonly

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

@@ -1,7 +1,5 @@
 [mediasource-duration.html]
   type: testharness
-  disabled:
-    if (os == "win") and (version != "5.1.2600"): https://bugzilla.mozilla.org/show_bug.cgi?id=1128332
   [Test seek starts on duration truncation below currentTime]
     expected: FAIL
 
@@ -12,7 +10,10 @@
       FAIL
 
   [Test endOfStream completes previous seek to truncated duration]
-    expected: FAIL
+    expected:
+      if (os == "win") and (version != "5.1.2600"): PASS
+      if os == "mac": PASS
+      FAIL
 
   [Test setting same duration multiple times does not fire duplicate durationchange]
     disabled: