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

chrome/browser/loader/chrome_resource_dispatcher_host_delegate.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/loader/chrome_resource_dispatcher_host_delegate.h"
 
 #include <stdint.h>
 #include <string>
 #include <utility>
 #include <vector>
@@ skipping 327 matching lines @@
     if (net_error == net::OK) {
       UMA_HISTOGRAM_LONG_TIMES("Net.NTP.Local.RequestTime2.Success",
                                request_loading_time);
     } else if (net_error == net::ERR_ABORTED) {
       UMA_HISTOGRAM_LONG_TIMES("Net.NTP.Local.RequestTime2.ErrAborted",
                                request_loading_time);
     }
   }
 }
 
+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,
     const content::GlobalRequestID& request_id,
     ResourceType resource_type,
     bool was_cached,
     bool used_data_reduction_proxy,
     int net_error,
     int64_t total_received_bytes,
     int64_t raw_body_bytes,
@@ skipping 78 matching lines @@
 void ChromeResourceDispatcherHostDelegate::RequestBeginning(
     net::URLRequest* request,
     content::ResourceContext* resource_context,
     content::AppCacheService* appcache_service,
     ResourceType resource_type,
     std::vector<std::unique_ptr<content::ResourceThrottle>>* throttles) {
   if (safe_browsing_.get())
     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);
 
 #if defined(OS_ANDROID)
   if (resource_type != content::RESOURCE_TYPE_MAIN_FRAME)
     InterceptNavigationDelegate::UpdateUserGestureCarryoverInfo(request);
 #endif
 
 #if defined(OS_CHROMEOS)
   // Check if we need to add merge session throttle. This throttle will postpone
@@ skipping 456 matching lines @@
           base::Bind(&ChromeResourceDispatcherHostDelegate::OnAbortedFrameLoad,
                     base::Unretained(this), url, request_loading_time));
       return;
   }
 
   std::string metric_name = (request_loading_time.InMilliseconds() < 100 ?
       "Net.ErrAborted.Fast" : "Net.ErrAborted.Slow");
   rappor::SampleDomainAndRegistryFromGURL(
       g_browser_process->rappor_service(), metric_name, url);
 }