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 336 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 GURL& url,

[RyanSturm, 2017/03/29 19:03:47]

Are you using url here?

[Pete Williamson, 2017/04/13 23:10:21]

Done.

+    const content::GlobalRequestID& request_id,
+    ResourceType resource_type,
+    base::TimeTicks request_creation_time) {
+  content::WebContents* web_contents = web_contents_getter.Run();
+
+  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 68 matching lines @@
         base::FieldTrialList::FindFullName("Prefetch");
     if (base::StartsWith(prefetch_experiment, "ExperimentDisable",
                          base::CompareCase::INSENSITIVE_ASCII)) {
       return false;
     }
   }
 
   return true;
 }
 
+// TODO: REVIEWERS: appcache_service is apparently not used.  Is there any
+// reason that I shouldn't remove it? (Maybe it's a part of some interface used
+// elsewhere.)
 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.
+  BrowserThread::PostTask(
+      BrowserThread::UI, FROM_HERE,
+      base::Bind(&NotifyUIThreadOfRequestStarted,
+                 info->GetWebContentsGetterForRequest(), request->url(),
+                 info->GetGlobalRequestID(), info->GetResourceType(),
+                 request->creation_time()));
+
 // The lowering of request priority causes issues with scheduling, since
 // 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::WebPageVisibilityStatePrerender;
   if (is_prerendering) {
@@ skipping 484 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);
 }