Wrap callbacks with v8::Locker.

In browser process, because of the use of v8::Locker in ProxyResolverV8,
creating HandleScope requires entering locker first, so in browser
process we should just use v8::Locker whenever we need to use V8 in a
callback called from non-V8 places (like dialog's callback, work pushed
to MessageLoop).

But also notice that the renderer process doesn't use v8::Locker, so we
have to be careful not to use it, otherwise blink would crash when
creating HandleScope.

browser/api/atom_api_window.cc

@@ -92,6 +92,7 @@ void Window::OnRendererCrashed() {
 
 void Window::OnCapturePageDone(const RefCountedV8Function& callback,
                                const std::vector<unsigned char>& data) {
+  v8::Locker locker(node_isolate);
   v8::HandleScope handle_scope(node_isolate);
 
   v8::Local<v8::Value> buffer = node::Buffer::New(