Bug 1128069: [MSE] P2. Change webref expectations now that the test is valid. r=jwwang

Test is failing on Linux Opt, see https://bugzilla.mozilla.org/show_bug.cgi?id=1128069
MozReview-Commit-ID: kaRQMtvEjN

--HG--
extra : rebase_source : 38cacead34e3bebdd8324af6faa5017ecaaff812

testing/web-platform/meta/media-source/mediasource-play-then-seek-back.html.ini

@@ -1,4 +1,6 @@
 [mediasource-play-then-seek-back.html]
   type: testharness
   prefs: [media.mediasource.enabled:true]
-  disabled: https://bugzilla.mozilla.org/show_bug.cgi?id=1128069
+  disabled:
+    if (os == "linux"): https://bugzilla.mozilla.org/show_bug.cgi?id=1293159
+