Bug 1611021 - part1 : add a method in special power to generate media control key events. r=bzbarsky

It allows us to simulate media control key events in non-privileged context, such as a mochitest running in content process.

Differential Revision: https://phabricator.services.mozilla.com/D60783

--HG--
extra : moz-landing-system : lando

testing/specialpowers/content/SpecialPowersChild.jsm

@@ -1293,6 +1293,10 @@ class SpecialPowersChild extends JSWindowActorChild {
     );
   }
 
+  async generateMediaControlKeyTestEvent(event) {
+    await this.sendQuery("SPGenerateMediaControlKeyTestEvent", { event });
+  }
+
   // Note: each call to registerConsoleListener MUST be paired with a
   // call to postConsoleSentinel; when the callback receives the
   // sentinel it will unregister itself (_after_ calling the

testing/specialpowers/content/SpecialPowersParent.jsm

@@ -1095,6 +1095,11 @@ class SpecialPowersParent extends JSWindowActorParent {
         return ServiceWorkerCleanUp.removeFromHost("example.com");
       }
 
+      case "SPGenerateMediaControlKeyTestEvent": {
+        ChromeUtils.generateMediaControlKeysTestEvent(aMessage.data.event);
+        return undefined;
+      }
+
       default:
         throw new SpecialPowersError(
           `Unrecognized Special Powers API: ${aMessage.name}`