[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 3e1ed690eb54a397b4dc557f18de0e0b39156b15..964600618beaa7d439609cf47649d857997620db 100644
--- a/content/public/browser/web_contents.h
+++ b/content/public/browser/web_contents.h
@@ -441,6 +441,9 @@ class WebContents : public PageNavigator,
   virtual void WasShown() = 0;
   virtual void WasHidden() = 0;
 
+  // Whether the WebContents is visible.
+  virtual bool IsVisible() const = 0;

[Charlie Reis, 2017/06/10 00:53:37]

I think there's probably a better way to check this.  Ken, do you know?

[kenrb, 2017/06/12 15:26:22]

The call site should be able to use:
navigation_handle->GetWebContents()->GetTopLevelRenderWidgetHostView()->IsShowing();

[nasko, 2017/06/12 23:22:02]

Doesn't that distribute implementation logic outside of WebContents? Is there a
downside to wrapping this call into a "IsShowing()" method on the WebContents
itself?

[Zhen Wang, 2017/06/13 23:33:21]

This is one option for us to know if a tab is visible. Since TabManager is also
observing TabStripModel, we can also track tab strip events explicitly.

But I think it is better to ask WebContents for this information directly. I
thought IsVisible() or IsShowing() would be some property WebContents should
provide. Is there any downside of adding this? 

By the way, is |should_normally_be_visible_| the same as
GetTopLevelRenderWidgetHostView()->IsShowing()?

[nasko, 2017/06/15 20:01:58]

I defer to kenrb@ on this one.

[kenrb, 2017/06/15 20:53:08]

Adding that method might make sense here. The only divergence between
should_normally_be_visible_ and RWHV::IsShowing(), as far as I am aware, is that
IsShowing() will return false while a top-level page is still loading, even
though should_normally_be_visible_ might return true. Given what you are using
it for, the latter seems like it is more relevant.

[Zhen Wang, 2017/06/19 23:00:12]

Right. We will need to know if it is visible before loading. For here, I will
keep the name |IsVisible()|, so to distinguish from the |IsShown()|. Let me know
if any new name is better.

+
   // 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