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..d76460bedc3da90816e3c529069cf540cfec0778 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(
+    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 +463,36 @@ 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.

[RyanSturm, 2017/04/17 19:57:08]

Maybe you should create a bug (if you haven't) and reference it in this comment.
Fixing this might get lost otherwise.

[Pete Williamson, 2017/04/17 20:46:36]

Done.

+  BrowserThread::PostTask(
+      BrowserThread::UI, FROM_HERE,
+      base::Bind(&NotifyUIThreadOfRequestStarted,
+                 info->GetWebContentsGetterForRequest(),
+                 info->GetGlobalRequestID(), info->GetResourceType(),
+                 request->creation_time()));
+
+// The lowering of request priority causes issues with scheduling, since

[RyanSturm, 2017/04/17 19:57:08]

This block seems like a bad re-base.

[Pete Williamson, 2017/04/17 20:46:36]

Good catch!  Removed.

+// content::ResourceScheduler uses it to delay and throttle requests. This is
+// disabled only on Android, as the prerenders are not likely to compete with
+// page loads there.
+// See https://crbug.com/652746 for details.
+// TODO(lizeb,droger): Fix the issue on all platforms.
+#if !defined(OS_ANDROID)
+  bool is_prerendering =
+      info->GetVisibilityState() == blink::kWebPageVisibilityStatePrerender;
+  if (is_prerendering) {
+    // Requests with the IGNORE_LIMITS flag set (i.e., sync XHRs)
+    // should remain at MAXIMUM_PRIORITY.
+    if (request->load_flags() & net::LOAD_IGNORE_LIMITS) {
+      DCHECK_EQ(request->priority(), net::MAXIMUM_PRIORITY);
+    } else {
+      request->SetPriority(net::IDLE);
+    }
+  }
+#endif  // OS_ANDROID
+
   ProfileIOData* io_data = ProfileIOData::FromResourceContext(
       resource_context);