Check whether WebContents lives before using its methods

Close #2064.

atom/browser/api/atom_api_web_contents.cc

@@ -792,6 +792,10 @@ mate::ObjectTemplateBuilder WebContents::GetObjectTemplateBuilder(
       isolate, v8::Local<v8::ObjectTemplate>::New(isolate, template_));
 }
 
+bool WebContents::IsDestroyed() const {
+  return !IsAlive();
+}
+
 void WebContents::OnRendererMessage(const base::string16& channel,
                                     const base::ListValue& args) {
   // webContents.emit(channel, new Event(), args...);

atom/browser/api/atom_api_web_contents.h

@@ -111,6 +111,7 @@ class WebContents : public mate::TrackableObject<WebContents>,
   // mate::Wrappable:
   mate::ObjectTemplateBuilder GetObjectTemplateBuilder(
       v8::Isolate* isolate) override;
+  bool IsDestroyed() const override;
 
   // content::WebContentsDelegate:
   bool AddMessageToConsole(content::WebContents* source,