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_browser_bindings.cc

@@ -25,6 +25,7 @@ void AtomBrowserBindings::OnRendererMessage(int process_id,
                                             int routing_id,
                                             const string16& channel,
                                             const base::ListValue& args) {
+  v8::Locker locker(node_isolate);
   v8::HandleScope handle_scope(node_isolate);
 
   scoped_ptr<V8ValueConverter> converter(new V8ValueConverter);
@@ -58,6 +59,7 @@ void AtomBrowserBindings::OnRendererMessageSync(
     const base::ListValue& args,
     NativeWindow* sender,
     IPC::Message* message) {
+  v8::Locker locker(node_isolate);
   v8::HandleScope handle_scope(node_isolate);
 
   scoped_ptr<V8ValueConverter> converter(new V8ValueConverter);