Reapply 169371 "DevTools: defer styles delta calculation to until the end of the task"

Source/web/WebDevToolsAgentImpl.cpp

Index: Source/web/WebDevToolsAgentImpl.cpp
diff --git a/Source/web/WebDevToolsAgentImpl.cpp b/Source/web/WebDevToolsAgentImpl.cpp
index 44305b61c853a28a67cf7da6f0fc1e535a46e0a1..452c9c17859528dda71436da251db4b33f73bab5 100644
--- a/Source/web/WebDevToolsAgentImpl.cpp
+++ b/Source/web/WebDevToolsAgentImpl.cpp
@@ -117,6 +117,11 @@ private:
             return;
         m_running = true;
 
+        // 0. Flush pending frontend messages.
+        WebViewImpl* viewImpl = WebViewImpl::fromPage(page);
+        WebDevToolsAgentImpl* agent = static_cast<WebDevToolsAgentImpl*>(viewImpl->devToolsAgent());
+        agent->flushPendingFrontendMessages();
+
         Vector<WebViewImpl*> views;
 
         // 1. Disable input events.
@@ -567,9 +572,7 @@ void WebDevToolsAgentImpl::sendMessageToFrontend(PassRefPtr<WebCore::JSONObject>
 
 void WebDevToolsAgentImpl::flush()
 {
-    for (size_t i = 0; i < m_frontendMessageQueue.size(); ++i)
-        m_client->sendMessageToInspectorFrontend(m_frontendMessageQueue[i]->toJSONString());
-    m_frontendMessageQueue.clear();
+    flushPendingFrontendMessages();
 }
 
 void WebDevToolsAgentImpl::updateInspectorStateCookie(const String& state)
@@ -603,18 +606,31 @@ void WebDevToolsAgentImpl::evaluateInWebInspector(long callId, const WebString&
     ic->evaluateForTestInFrontend(callId, script);
 }
 
+void WebDevToolsAgentImpl::flushPendingFrontendMessages()
+{
+    InspectorController* ic = inspectorController();
+    ic->flushPendingFrontendMessages();
+
+    for (size_t i = 0; i < m_frontendMessageQueue.size(); ++i)
+        m_client->sendMessageToInspectorFrontend(m_frontendMessageQueue[i]->toJSONString());
+    m_frontendMessageQueue.clear();
+}
+
 void WebDevToolsAgentImpl::willProcessTask()
 {
+    if (!m_attached)
+        return;
     if (InspectorController* ic = inspectorController())
         ic->willProcessTask();
 }
 
 void WebDevToolsAgentImpl::didProcessTask()
 {
+    if (!m_attached)
+        return;
     if (InspectorController* ic = inspectorController())
         ic->didProcessTask();
-    if (m_attached)
-        flush();
+    flushPendingFrontendMessages();
 }
 
 WebString WebDevToolsAgent::inspectorProtocolVersion()