Add MainFrameResource.NavigationStartToReceiveHeadersStart UKM metric.

This metric will be used for a time to first byte metric in
Chrome User Experience Report.

UKM collection review: https://docs.google.com/document/d/1U0gT3RalytZqPW9t-4lBweBwLaamXYdJaO4ETsuvCdU/edit#heading=h.k5jx6iluw4yt

Change-Id: Ie311d28f98b90f4361845e10843fa50f066c32b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628048Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Bryan McQuade <bmcquade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663169}

chrome/browser/page_load_metrics/observers/ukm_page_load_metrics_observer.cc

@@ -520,6 +520,15 @@ void UkmPageLoadMetricsObserver::ReportMainResourceTimingMetrics(
         navigation_start_to_request_start.InMilliseconds());
   }
 
+  if (!main_frame_timing_->receive_headers_start.is_null() &&
+      !GetDelegate()->GetNavigationStart().is_null()) {
+    base::TimeDelta navigation_start_to_receive_headers_start =
+        main_frame_timing_->receive_headers_start -
+        GetDelegate()->GetNavigationStart();
+    builder->SetMainFrameResource_NavigationStartToReceiveHeadersStart(
+        navigation_start_to_receive_headers_start.InMilliseconds());
+  }
+
   if (connection_info_.has_value()) {
     page_load_metrics::NetworkProtocol protocol =
         page_load_metrics::GetNetworkProtocol(*connection_info_);

tools/metrics/ukm/ukm.xml

@@ -4998,6 +4998,12 @@ be describing additional metrics about the same event.
       tools/metrics/histograms/enum.xml.
     </summary>
   </metric>
+  <metric name="MainFrameResource.NavigationStartToReceiveHeadersStart">
+    <summary>
+      The duration from the start of the navigation to the start of the response
+      headers being received. In ms.
+    </summary>
+  </metric>
   <metric name="MainFrameResource.NavigationStartToRequestStart">
     <summary>
       The duration from the start of the navigation to the start of the main