DevTools: use per-LocalFrame instrumenting agents instead of per-Page ones.

Source/core/inspector/InspectorPageAgent.h

Index: Source/core/inspector/InspectorPageAgent.h
diff --git a/Source/core/inspector/InspectorPageAgent.h b/Source/core/inspector/InspectorPageAgent.h
index 7170cc4a1435a579d2503176a490588c495559e9..cc850c4eec9b9bf9a2bf6ef794c71fb893d35803 100644
--- a/Source/core/inspector/InspectorPageAgent.h
+++ b/Source/core/inspector/InspectorPageAgent.h
@@ -43,7 +43,7 @@ namespace blink {
 class Resource;
 class Document;
 class DocumentLoader;
-class LocalFrame;
+class FrameHost;
 class GraphicsContext;
 class GraphicsLayer;
 class InjectedScriptManager;
@@ -52,6 +52,7 @@ class InspectorOverlay;
 class InspectorResourceContentLoader;
 class KURL;
 class LayoutRect;
+class LocalFrame;
 class Page;
 class RenderObject;
 class SharedBuffer;
@@ -157,8 +158,8 @@ public:
     void discardAgent() override;
 
     // Cross-agents API
-    Page* page() { return m_page; }
-    LocalFrame* mainFrame();
+    FrameHost* frameHost();
+    LocalFrame* inspectedFrame();
     String createIdentifier();
     LocalFrame* frameForId(const String& frameId);
     String frameId(LocalFrame*);