[page_load_metrics] Add a NavigationThrottle for richer abort metrics

content/browser/frame_host/navigation_handle_impl.cc

Index: content/browser/frame_host/navigation_handle_impl.cc
diff --git a/content/browser/frame_host/navigation_handle_impl.cc b/content/browser/frame_host/navigation_handle_impl.cc
index 47dc9a23962c4e42ceaede2f50cd7418f4283fe0..0bc1ceb31c46c5a8bc6cfa7f0cd1684ed024c000 100644
--- a/content/browser/frame_host/navigation_handle_impl.cc
+++ b/content/browser/frame_host/navigation_handle_impl.cc
@@ -552,15 +552,16 @@ void NavigationHandleImpl::RegisterNavigationThrottles() {
   // RegisterThrottleForTesting.
   ScopedVector<NavigationThrottle> throttles_to_register =
       GetContentClient()->browser()->CreateThrottlesForNavigation(this);
+  std::unique_ptr<NavigationThrottle> devtools_throttle =
+      RenderFrameDevToolsAgentHost::CreateThrottleForNavigation(this);

[nasko, 2016/08/01 15:29:23]

This throttle was added at the end of the list intentionally. Are we sure it is
ok to move before the rest?

[Charlie Harrison, 2016/08/01 17:42:19]

It is still at the end of the "real" throttles. The only throttles that can come
after it are ones from RegisterThrottleForTesting. If a time comes when a
testing throttles needs to come before a real throttle, I think we can add
support for it then.

[nasko, 2016/08/01 23:38:11]

If it has no effect, then what necessitated the change?

[Charlie Harrison, 2016/08/02 13:09:14]

The code moves the test navigation throttles to the end, so that
TestNavigationManager defers a navigation after the page_load_metrics system
observes it. Otherwise we can't use TestNavigationManager to test the new code
path.

+  if (devtools_throttle)
+    throttles_to_register.push_back(devtools_throttle.release());
+
   if (throttles_to_register.size() > 0) {
-    throttles_.insert(throttles_.end(), throttles_to_register.begin(),
+    throttles_.insert(throttles_.begin(), throttles_to_register.begin(),
                       throttles_to_register.end());
     throttles_to_register.weak_clear();
   }
-  std::unique_ptr<NavigationThrottle> devtools_throttle =
-      RenderFrameDevToolsAgentHost::CreateThrottleForNavigation(this);
-  if (devtools_throttle)
-    throttles_.push_back(devtools_throttle.release());
 }
 
 }  // namespace content