Add CSSTiming to collect aggregate PLT-level stats about CSS.

third_party/WebKit/Source/core/css/CSSTiming.cpp

+// 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 "core/css/CSSTiming.h"
+
+#include "core/paint/PaintTiming.h"
+
+namespace blink {
+
+static const char kSupplementName[] = "CSSTiming";
+
+CSSTiming& CSSTiming::from(Document& document) {
+  CSSTiming* timing = static_cast<CSSTiming*>(
+      Supplement<Document>::from(document, kSupplementName));
+  if (!timing) {
+    timing = new CSSTiming(document);
+    Supplement<Document>::provideTo(document, kSupplementName, timing);
+  }
+  return *timing;
+}
+
+void CSSTiming::recordAuthorStyleSheetParseTime(double seconds) {
+  if (!m_paintTiming->firstContentfulPaint())
+    m_parseTimeBeforeFCP += seconds;
+}
+
+DEFINE_TRACE(CSSTiming) {
+  visitor->trace(m_document);
+  visitor->trace(m_paintTiming);
+  Supplement<Document>::trace(visitor);
+}
+
+CSSTiming::CSSTiming(Document& document)
+    : m_document(document), m_paintTiming(PaintTiming::from(document)) {}
+
+}  // namespace blink