Add page_load_metrics for css scanning experiment

chrome/browser/page_load_metrics/observers/css_scanning_page_load_metrics_observer.cc

Index: chrome/browser/page_load_metrics/observers/css_scanning_page_load_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/observers/css_scanning_page_load_metrics_observer.cc b/chrome/browser/page_load_metrics/observers/css_scanning_page_load_metrics_observer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..adc54abcd938033a6e054e328b425fcfca63560e
--- /dev/null
+++ b/chrome/browser/page_load_metrics/observers/css_scanning_page_load_metrics_observer.cc
@@ -0,0 +1,26 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/page_load_metrics/observers/css_scanning_page_load_metrics_observer.h"
+
+#include "chrome/browser/page_load_metrics/page_load_metrics_util.h"
+#include "third_party/WebKit/public/platform/WebLoadingBehaviorFlag.h"
+
+CssScanningMetricsObserver::CssScanningMetricsObserver() {}
+
+CssScanningMetricsObserver::~CssScanningMetricsObserver() {}
+
+void CssScanningMetricsObserver::OnFirstMeaningfulPaint(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& info) {
+  if (info.metadata.behavior_flags &
+          blink::WebLoadingBehaviorFlag::WebLoadingBehaviorCSSPreloadFound &&
+      WasStartedInForegroundOptionalEventInForeground(
+          timing.first_meaningful_paint, info)) {
+    PAGE_LOAD_HISTOGRAM(
+        "PageLoad.Clients.CssScanner.Experimental.PaintTiming"
+        "ParseStartToFirstMeaningfulPaint",
+        timing.first_meaningful_paint.value() - timing.parse_start.value());
+  }
+}