Avoid using declarations in headers which introduce symbol conflicts.

third_party/WebKit/Source/core/inspector/InspectorInputAgent.h

Index: third_party/WebKit/Source/core/inspector/InspectorInputAgent.h
diff --git a/third_party/WebKit/Source/core/inspector/InspectorInputAgent.h b/third_party/WebKit/Source/core/inspector/InspectorInputAgent.h
index 3917428b6fae37bcdab2a1253bf1de328f2c4535..0c830dac23699122867d90a62e2c11fb836223ec 100644
--- a/third_party/WebKit/Source/core/inspector/InspectorInputAgent.h
+++ b/third_party/WebKit/Source/core/inspector/InspectorInputAgent.h
@@ -53,11 +53,11 @@ class CORE_EXPORT InspectorInputAgent final
   DECLARE_VIRTUAL_TRACE();
 
   // Methods called from the frontend for simulating input.
-  Response dispatchTouchEvent(
+  protocol::Response dispatchTouchEvent(
       const String& type,
       std::unique_ptr<protocol::Array<protocol::Input::TouchPoint>> touchPoints,
-      Maybe<int> modifiers,
-      Maybe<double> timestamp) override;
+      protocol::Maybe<int> modifiers,
+      protocol::Maybe<double> timestamp) override;
 
  private:
   explicit InspectorInputAgent(InspectedFrames*);