service worker: Remove expired histogram for nav preload header size.

Bug: 1089495
Change-Id: I8af3f627d076a03a58c61d183f63216ac31add79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2226000Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774377}

content/browser/service_worker/service_worker_fetch_dispatcher.cc

@@ -706,8 +706,6 @@ bool ServiceWorkerFetchDispatcher::MaybeStartNavigationPreload(
 
   DCHECK(net::HttpUtil::IsValidHeaderValue(
       version_->navigation_preload_state().header));
-  ServiceWorkerMetrics::RecordNavigationPreloadRequestHeaderSize(
-      version_->navigation_preload_state().header.length());
   resource_request.headers.SetHeader(
       "Service-Worker-Navigation-Preload",
       version_->navigation_preload_state().header);

content/browser/service_worker/service_worker_metrics.cc

@@ -543,12 +543,6 @@ void ServiceWorkerMetrics::RecordStartStatusAfterFailure(
   }
 }
 
-void ServiceWorkerMetrics::RecordNavigationPreloadRequestHeaderSize(
-    size_t size) {
-  UMA_HISTOGRAM_COUNTS_100000("ServiceWorker.NavigationPreload.HeaderSize",
-                              size);
-}
-
 void ServiceWorkerMetrics::RecordRuntime(base::TimeDelta time) {
   // Start at 1 second since we expect service worker to last at least this
   // long: the update timer and idle timeout timer run on the order of seconds.

tools/metrics/histograms/histograms.xml

@@ -151904,6 +151904,9 @@ should be kept until we remove incident reporting. -->
 
 <histogram name="ServiceWorker.NavigationPreload.HeaderSize" units="bytes"
     expires_after="M85">
+  <obsolete>
+    Expired June 2020.
+  </obsolete>
   <owner>horo@chromium.org</owner>
   <summary>
     The size of Service-Worker-Navigation-Preload header when the navigation