[TooManyTabs] Add TabNavigationThrottle

content/public/browser/web_contents.h

Index: content/public/browser/web_contents.h
diff --git a/content/public/browser/web_contents.h b/content/public/browser/web_contents.h
index 5881c954de4880deed9f61998c4c730e457b0199..993bf7a4934c1847792a8ea6ae15a75e1f57a32a 100644
--- a/content/public/browser/web_contents.h
+++ b/content/public/browser/web_contents.h
@@ -442,6 +442,11 @@ class WebContents : public PageNavigator,
   virtual void WasShown() = 0;
   virtual void WasHidden() = 0;
 
+  // Whether the WebContents is visible. This can return true even if the page
+  // is still loading, as opposed to RenderWidgetHostView::IsShowing(), which
+  // always returns false when the page is still loading.
+  virtual bool IsVisible() const = 0;
+
   // Returns true if the before unload and unload listeners need to be
   // fired. The value of this changes over time. For example, if true and the
   // before unload listener is executed and allows the user to exit, then this