Avoid using declarations in headers which introduce symbol conflicts.

third_party/WebKit/Source/web/InspectorEmulationAgent.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "web/InspectorEmulationAgent.h"
 
 #include "core/frame/FrameHost.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/Settings.h"
 #include "core/inspector/protocol/DOM.h"
 #include "core/page/Page.h"
 #include "platform/geometry/DoubleRect.h"
 #include "platform/graphics/Color.h"
 #include "public/platform/Platform.h"
 #include "public/platform/WebFloatPoint.h"
 #include "public/platform/WebThread.h"
 #include "public/platform/WebViewScheduler.h"
 #include "web/DevToolsEmulator.h"
 #include "web/WebLocalFrameImpl.h"
 #include "web/WebViewImpl.h"
 
 namespace blink {
 
+using protocol::Maybe;
+using protocol::Response;
+
 namespace EmulationAgentState {
 static const char scriptExecutionDisabled[] = "scriptExecutionDisabled";
 static const char touchEventEmulationEnabled[] = "touchEventEmulationEnabled";
 static const char emulatedMedia[] = "emulatedMedia";
 static const char forcedViewportEnabled[] = "forcedViewportEnabled";
 static const char forcedViewportX[] = "forcedViewportX";
 static const char forcedViewportY[] = "forcedViewportY";
 static const char forcedViewportScale[] = "forcedViewportScale";
 static const char defaultBackgroundColorOverrideRGBA[] =
     "defaultBackgroundColorOverrideRGBA";
@@ skipping 168 matching lines @@
       Color(rgba->getR(), rgba->getG(), rgba->getB(), alpha).rgb());
   return Response::OK();
 }
 
 DEFINE_TRACE(InspectorEmulationAgent) {
   visitor->trace(m_webLocalFrameImpl);
   InspectorBaseAgent::trace(visitor);
 }
 
 }  // namespace blink