Flush frontend messages after a binding is called

Without flushing the messages, the timing of the notifications
is unpredictable making it hard to build features on top.

Bug: chromium:1224371
Change-Id: Ice09ef82231186d1a93ca82508c49e26585ce260
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2991237Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75525}

src/inspector/v8-runtime-agent-impl.cc

@@ -799,6 +799,7 @@ void V8RuntimeAgentImpl::bindingCalled(const String16& name,
                                        int executionContextId) {
   if (!m_activeBindings.count(name)) return;
   m_frontend.bindingCalled(name, payload, executionContextId);
+  m_frontend.flush();
 }
 
 void V8RuntimeAgentImpl::addBindings(InspectedContext* context) {