Bug 1147699 follow-up: Disable the plugin test on Mulet

Landed on a CLOSED TREE in order to fix the test bustage.

dom/workers/test/serviceworkers/fetch/context/index.html

@@ -226,7 +226,7 @@
 
   function testPlugin() {
     var isMobile = /Mobile|Tablet/.test(navigator.userAgent);
-    if (isMobile) {
+    if (isMobile || parent.isMulet()) {
       // We can't use plugins on mobile, so skip this part of the test there.
       return Promise.resolve();
     }

dom/workers/test/serviceworkers/test_request_context.html

@@ -49,6 +49,14 @@
   }
   setTestPluginEnabledState(SpecialPowers.Ci.nsIPluginTag.STATE_ENABLED);
 
+  function isMulet() {
+    try {
+      return SpecialPowers.getBoolPref("b2g.is_mulet");
+    } catch(e) {
+      return false;
+    }
+  }
+
   var iframe;
   function runTest() {
     iframe = document.querySelector("iframe");