Add ServiceWorker.NavigationPreload.HeaderSize UMA

BUG=649558

Review-Url: https://codereview.chromium.org/2626943002
Cr-Commit-Position: refs/heads/master@{#443086}

content/browser/service_worker/service_worker_fetch_dispatcher.cc

@@ -17,6 +17,7 @@
 #include "content/browser/loader/resource_requester_info.h"
 #include "content/browser/loader/url_loader_factory_impl.h"
 #include "content/browser/service_worker/embedded_worker_status.h"
+#include "content/browser/service_worker/service_worker_metrics.h"
 #include "content/browser/service_worker/service_worker_version.h"
 #include "content/common/service_worker/service_worker_event_dispatcher.mojom.h"
 #include "content/common/service_worker/service_worker_messages.h"
@@ -448,6 +449,8 @@ void ServiceWorkerFetchDispatcher::MaybeStartNavigationPreload(
 
   DCHECK(net::HttpUtil::IsValidHeaderValue(
       version_->navigation_preload_state().header));
+  ServiceWorkerMetrics::RecordNavigationPreloadRequestHeaderSize(
+      version_->navigation_preload_state().header.length());
   request.headers = "Service-Worker-Navigation-Preload: " +
                     version_->navigation_preload_state().header;
 

content/browser/service_worker/service_worker_metrics.cc

@@ -761,4 +761,10 @@ void ServiceWorkerMetrics::RecordStartStatusAfterFailure(
   }
 }
 
+void ServiceWorkerMetrics::RecordNavigationPreloadRequestHeaderSize(
+    size_t size) {
+  UMA_HISTOGRAM_COUNTS_100000("ServiceWorker.NavigationPreload.HeaderSize",
+                              size);
+}
+
 }  // namespace content

content/browser/service_worker/service_worker_metrics.h

@@ -286,6 +286,10 @@ class ServiceWorkerMetrics {
   static void RecordStartStatusAfterFailure(int failure_count,
                                             ServiceWorkerStatusCode status);
 
+  // Records the size of Service-Worker-Navigation-Preload header when the
+  // navigation preload request is to be sent.
+  static void RecordNavigationPreloadRequestHeaderSize(size_t size);
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(ServiceWorkerMetrics);
 };

tools/metrics/histograms/histograms.xml

@@ -59599,6 +59599,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ServiceWorker.NavigationPreload.HeaderSize" units="bytes">
+  <owner>horo@chromium.org</owner>
+  <summary>
+    The size of Service-Worker-Navigation-Preload header when the navigation
+    preload request is to be sent. The default value of the header is
+    &quot;true&quot;, so the default size is 4.
+  </summary>
+</histogram>
+
 <histogram name="ServiceWorker.NotificationClickEvent.Time" units="ms">
   <owner>peter@chromium.org</owner>
   <summary>