Send an event to the page load metrics to track resource starting.

chrome/browser/loader/chrome_resource_dispatcher_host_delegate.cc

Index: chrome/browser/loader/chrome_resource_dispatcher_host_delegate.cc
diff --git a/chrome/browser/loader/chrome_resource_dispatcher_host_delegate.cc b/chrome/browser/loader/chrome_resource_dispatcher_host_delegate.cc
index cc36f63584bf41e131932889a297f929981c2f81..01064a142426c83f0558f469b02f750806cfa22a 100644
--- a/chrome/browser/loader/chrome_resource_dispatcher_host_delegate.cc
+++ b/chrome/browser/loader/chrome_resource_dispatcher_host_delegate.cc
@@ -345,6 +345,26 @@ void LogMainFrameMetricsOnUIThread(const GURL& url,
   }
 }
 
+void NotifyUIThreadOfRequestStarted(

[Charlie Harrison, 2017/04/18 16:51:32]

Can we use WebContentsObserver::DidGetResourceResponseStart instead?

[RyanSturm, 2017/04/18 17:23:57]

This seems to be called when headers are received. I think that won't track the
pending requests very well considering we can only have 6 sockets open to the
same host (I'm assuming each request takes 1 socket, but I'm not sure). Suppose
the page wanted to start 100 image requests, when 12 finish it would look like
12/18 are done, which might seem good enough. WDY?T

https://cs.chromium.org/chromium/src/net/socket/client_socket_pool_manager.cc...

[Charlie Harrison, 2017/04/18 17:38:35]

Woops, you're right I completely misread the name :P

+    const content::ResourceRequestInfo::WebContentsGetter& web_contents_getter,
+    const content::GlobalRequestID& request_id,
+    ResourceType resource_type,
+    base::TimeTicks request_creation_time) {
+  content::WebContents* web_contents = web_contents_getter.Run();
+
+  if (!web_contents)
+    return;
+
+  page_load_metrics::MetricsWebContentsObserver* metrics_observer =
+      page_load_metrics::MetricsWebContentsObserver::FromWebContents(
+          web_contents);
+
+  if (metrics_observer) {
+    metrics_observer->OnRequestStarted(request_id, resource_type,
+                                       request_creation_time);
+  }
+}
+
 void NotifyUIThreadOfRequestComplete(
     const content::ResourceRequestInfo::WebContentsGetter& web_contents_getter,
     const GURL& url,
@@ -443,6 +463,17 @@ void ChromeResourceDispatcherHostDelegate::RequestBeginning(
     safe_browsing_->OnResourceRequest(request);
 
   const ResourceRequestInfo* info = ResourceRequestInfo::ForRequest(request);
+
+  // TODO(petewil): Unify the safe browsing request and the metrics observer
+  // request if possible so we only have to cross to the main thread once.
+  // http://crbug.com/712312.
+  BrowserThread::PostTask(
+      BrowserThread::UI, FROM_HERE,
+      base::Bind(&NotifyUIThreadOfRequestStarted,
+                 info->GetWebContentsGetterForRequest(),
+                 info->GetGlobalRequestID(), info->GetResourceType(),
+                 request->creation_time()));
+
   ProfileIOData* io_data = ProfileIOData::FromResourceContext(
       resource_context);