Revert of Remove the is_loading_ field from WebContentsImpl

content/browser/site_instance_impl.h

Index: content/browser/site_instance_impl.h
diff --git a/content/browser/site_instance_impl.h b/content/browser/site_instance_impl.h
index 81967cfffa12d827d3e8c35687ee63d9c6ac4fed..b54de7f5886673ad1cf39193233641b751121df4 100644
--- a/content/browser/site_instance_impl.h
+++ b/content/browser/site_instance_impl.h
@@ -23,11 +23,11 @@
 class CONTENT_EXPORT SiteInstanceImpl : public SiteInstance,
                                         public RenderProcessHostObserver {
  public:
-  class CONTENT_EXPORT Observer {
+  class Observer {
    public:
     // Called when this SiteInstance transitions to having no active frames,
     // as measured by active_frame_count().
-    virtual void ActiveFrameCountIsZero(SiteInstanceImpl* site_instance) {}
+    virtual void ActiveFrameCountIsZero(SiteInstanceImpl* site_instance) = 0;
 
     // Called when the renderer process of this SiteInstance has exited.
     virtual void RenderProcessGone(SiteInstanceImpl* site_instance) = 0;