Bug 1543098 - Implement Runtime.executionContextDestroyed. r=ato

Differential Revision: https://phabricator.services.mozilla.com/D27524

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

remote/domains/content/Runtime.jsm

@@ -27,6 +27,8 @@ class Runtime extends ContentProcessDomain {
       this.chromeEventHandler.addEventListener("DOMWindowCreated", this,
         {mozSystemGroup: true});
 
+      Services.obs.addObserver(this, "inner-window-destroyed");
+
       // Spin the event loop in order to send the `executionContextCreated` event right
       // after we replied to `enable` request.
       Services.tm.dispatchToMainThread(() => {
@@ -50,6 +52,7 @@ class Runtime extends ContentProcessDomain {
       this.enabled = false;
       this.chromeEventHandler.removeEventListener("DOMWindowCreated", this,
         {mozSystemGroup: true});
+      Services.obs.removeObserver(this, "inner-window-destroyed");
     }
   }
 
@@ -70,4 +73,11 @@ class Runtime extends ContentProcessDomain {
       break;
     }
   }
+
+  observe(subject, topic, data) {
+    const innerWindowID = subject.QueryInterface(Ci.nsISupportsPRUint64).data;
+    this.emit("Runtime.executionContextDestroyed", {
+      executionContextId: innerWindowID,
+    });
+  }
 }