Remove WebCore.PreloadDelay histogram

Bug: 768636
Change-Id: Iaa49f32a16a57c079c6aaaf4fd3ba84756aa88a7
Reviewed-on: https://chromium-review.googlesource.com/701139Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506634}

third_party/WebKit/Source/core/html/parser/HTMLResourcePreloader.cpp

@@ -30,10 +30,8 @@
 #include "core/frame/Settings.h"
 #include "core/loader/DocumentLoader.h"
 #include "core/loader/resource/CSSStyleSheetResource.h"
-#include "platform/Histogram.h"
 #include "platform/loader/fetch/Resource.h"
 #include "platform/loader/fetch/ResourceFetcher.h"
-#include "public/platform/Platform.h"
 #include <memory>
 
 namespace blink {
@@ -78,12 +76,6 @@ void HTMLResourcePreloader::Preload(
   if (!document_->Loader())
     return;
 
-  int duration = static_cast<int>(
-      1000 * (MonotonicallyIncreasingTime() - preload->DiscoveryTime()));
-  DEFINE_STATIC_LOCAL(CustomCountHistogram, preload_delay_histogram,
-                      ("WebCore.PreloadDelayMs", 0, 2000, 20));
-  preload_delay_histogram.Count(duration);
-
   Resource* resource = preload->Start(document_);
 
   if (resource && !resource->IsLoaded() &&

third_party/WebKit/Source/core/html/parser/PreloadRequest.h

@@ -69,7 +69,6 @@ class CORE_EXPORT PreloadRequest {
 
   Resource* Start(Document*);
 
-  double DiscoveryTime() const { return discovery_time_; }
   void SetDefer(FetchParameters::DeferOption defer) { defer_ = defer; }
   void SetCharset(const String& charset) { charset_ = charset.IsolatedCopy(); }
   void SetCrossOrigin(CrossOriginAttributeValue cross_origin) {

tools/metrics/histograms/histograms.xml

@@ -90558,6 +90558,10 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="WebCore.PreloadDelayMs" units="ms">
+  <obsolete>
+    Deprecated, preloads are issued on the main thread now, so this is less
+    useful.
+  </obsolete>
   <owner>csharrison@chromium.org</owner>
   <summary>
     The delay between when the preload scanner discovers a resource on the