Remove Blink.ResourceFetcher.StaleWhileRevalidateDuration metric.

The metric has been expired. Remove dead code.

BUG=1055384

Change-Id: Ic5df4f8f54adc8057993ab70e66403125e4a5914
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587433Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836835}

third_party/blink/renderer/platform/loader/fetch/stale_revalidation_resource_client.cc

@@ -11,7 +11,7 @@ namespace blink {
 
 StaleRevalidationResourceClient::StaleRevalidationResourceClient(
     Resource* stale_resource)
-    : start_time_(base::TimeTicks::Now()), stale_resource_(stale_resource) {}
+    : stale_resource_(stale_resource) {}
 
 StaleRevalidationResourceClient::~StaleRevalidationResourceClient() = default;
 
@@ -20,13 +20,6 @@ void StaleRevalidationResourceClient::NotifyFinished(Resource* resource) {
   if (stale_resource_ && IsMainThread())
     GetMemoryCache()->Remove(stale_resource_);
   ClearResource();
-
-  base::TimeTicks response_end = resource->LoadResponseEnd();
-  if (!response_end.is_null()) {
-    UMA_HISTOGRAM_LONG_TIMES(
-        "Blink.ResourceFetcher.StaleWhileRevalidateDuration",
-        response_end - start_time_);
-  }
 }
 
 void StaleRevalidationResourceClient::Trace(Visitor* visitor) const {

third_party/blink/renderer/platform/loader/fetch/stale_revalidation_resource_client.h

@@ -26,7 +26,6 @@ class StaleRevalidationResourceClient
   String DebugName() const override;
 
  private:
-  base::TimeTicks start_time_;
   // |stale_resource_| is the original resource that will be removed from the
   // MemoryCache when this revalidation request is completed. Note that it is
   // different than the active resource for this resource client which accessed

tools/metrics/histograms/histograms_xml/blink/histograms.xml

@@ -1740,6 +1740,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Blink.ResourceFetcher.StaleWhileRevalidateDuration" units="ms"
     expires_after="2020-04-05">
+  <obsolete>
+    Removed 12/2020.
+  </obsolete>
   <owner>dtapuska@chromium.org</owner>
   <owner>kenjibaheux@google.com</owner>
   <summary>Duration of completed stale revalidation attempts.</summary>