Enable flutter service protocol rpcs to run on UI isolate. (flutter#5263

)

With this change if flutter tools provides isolateId parameter, then VM will run the handler on that isolate. If no isolateId is provided, VM will continue running handlers as if they were registered wit Dart_RegisterRootServiceRequestCallback.

This goes towards fixing flutter#17434

runtime/service_protocol.cc

@@ -63,7 +63,7 @@ void ServiceProtocol::RemoveHandler(Handler* handler) {
 
 void ServiceProtocol::ToggleHooks(bool set) {
   for (const auto& endpoint : endpoints_) {
-    Dart_RegisterRootServiceRequestCallback(
+    Dart_RegisterIsolateServiceRequestCallback(
         endpoint.data(),                  // method
         &ServiceProtocol::HandleMessage,  // callback
         set ? this : nullptr              // user data