Convert dom_distiller::WebContentsMainFrameObserver to use the new navigation callbacks.

content/test/web_contents_observer_sanity_checker.cc

Index: content/test/web_contents_observer_sanity_checker.cc
diff --git a/content/test/web_contents_observer_sanity_checker.cc b/content/test/web_contents_observer_sanity_checker.cc
index ae9af67dc0e60f3940fa47bb76bd591838b4b08f..c84c2712304fefc64d65249fe38368116e6af14e 100644
--- a/content/test/web_contents_observer_sanity_checker.cc
+++ b/content/test/web_contents_observer_sanity_checker.cc
@@ -127,7 +127,8 @@ void WebContentsObserverSanityChecker::RenderFrameHostChanged(
 
   EnsureStableParentValue(new_host);
   if (new_host->GetParent()) {
-    AssertRenderFrameExists(new_host->GetParent());
+    //
+    //AssertRenderFrameExists(new_host->GetParent());

[nyquist, 2017/02/01 07:10:04]

It looks from the comments in the test code that the change to this file should
be scrapped now?

[jam, 2017/02/01 15:31:04]

Ooops, thanks fixed.

     CHECK(current_hosts_.count(GetRoutingPair(new_host->GetParent())))
         << "Parent of frame being committed must be current.";
   }