Revert of Remove the is_loading_ field from WebContentsImpl

content/browser/frame_host/render_frame_host_impl.h

Index: content/browser/frame_host/render_frame_host_impl.h
diff --git a/content/browser/frame_host/render_frame_host_impl.h b/content/browser/frame_host/render_frame_host_impl.h
index a4ba3a463d8af5ecfd69530c00361628e6e795a8..9767a4a5018b459a68f507ec47e21d41499c7abd 100644
--- a/content/browser/frame_host/render_frame_host_impl.h
+++ b/content/browser/frame_host/render_frame_host_impl.h
@@ -96,9 +96,9 @@
   CREATE_RF_HIDDEN = 1 << 1,
 };
 
-class CONTENT_EXPORT RenderFrameHostImpl : public RenderFrameHost,
-                                           public BrowserAccessibilityDelegate,
-                                           public SiteInstanceImpl::Observer {
+class CONTENT_EXPORT RenderFrameHostImpl
+    : public RenderFrameHost,
+      public BrowserAccessibilityDelegate {
  public:
   using AXTreeSnapshotCallback =
       base::Callback<void(
@@ -207,9 +207,6 @@
   gfx::AcceleratedWidget AccessibilityGetAcceleratedWidget() override;
   gfx::NativeViewAccessible AccessibilityGetNativeViewAccessible() override;
 
-  // SiteInstanceImpl::Observer
-  void RenderProcessGone(SiteInstanceImpl* site_instance) override;
-
   // Creates a RenderFrame in the renderer process.
   bool CreateRenderFrame(int proxy_routing_id,
                          int opener_routing_id,
@@ -249,12 +246,6 @@
   // FrameTreeNode. The proper way to check whether a frame is loading is to
   // call FrameTreeNode::IsLoading.
   bool is_loading() const { return is_loading_; }
-
-  // Sets this RenderFrameHost loading state. This is only used in the case of
-  // transfer navigations, where no DidStart/DidStopLoading notifications
-  // should be sent during the transfer.
-  // TODO(clamy): Remove this once PlzNavigate ships.
-  void set_is_loading(bool is_loading) { is_loading_ = is_loading; }
 
   // This returns the RenderFrameHost's owned RenderWidgetHost if it has one,
   // or else it returns nullptr.
@@ -535,10 +526,6 @@
 
   // Returns the Mojo ImageDownloader service.
   const image_downloader::ImageDownloaderPtr& GetMojoImageDownloader();
-
-  // Resets the loading state. Following this call, the RenderFrameHost will be
-  // in a non-loading state.
-  void ResetLoadingState();
 
  protected:
   friend class RenderFrameHostFactory;
@@ -585,6 +572,7 @@
       const base::string16& error_description,
       bool was_ignored_by_handler);
   void OnDidCommitProvisionalLoad(const IPC::Message& msg);
+  void OnDidDropNavigation();
   void OnUpdateState(const PageState& state);
   void OnBeforeUnloadACK(
       bool proceed,