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_dialog.cc

@@ -19,7 +19,9 @@ namespace {
 
 template<typename T>
 void CallV8Function(const RefCountedV8Function& callback, T arg) {
+  v8::Locker locker(node_isolate);
   v8::HandleScope handle_scope(node_isolate);
+
   v8::Handle<v8::Value> value = ToV8Value(arg);
   callback->NewHandle(node_isolate)->Call(
       v8::Context::GetCurrent()->Global(), 1, &value);