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

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

Index: third_party/WebKit/Source/core/css/CSSTiming.h
diff --git a/third_party/WebKit/Source/core/css/CSSTiming.h b/third_party/WebKit/Source/core/css/CSSTiming.h
new file mode 100644
index 0000000000000000000000000000000000000000..73cce247e24a23410c1a06669b52164e20e8adbc
--- /dev/null
+++ b/third_party/WebKit/Source/core/css/CSSTiming.h
@@ -0,0 +1,48 @@
+// 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.
+
+#ifndef CSSTiming_h
+#define CSSTiming_h
+
+#include "core/dom/Document.h"
+#include "platform/Supplementable.h"
+#include "platform/heap/Handle.h"
+#include "wtf/Noncopyable.h"
+
+namespace blink {
+
+class PaintTiming;
+
+class CSSTiming : public GarbageCollectedFinalized<CSSTiming>,
+                  public Supplement<Document> {
+  WTF_MAKE_NONCOPYABLE(CSSTiming);
+  USING_GARBAGE_COLLECTED_MIXIN(CSSTiming);
+
+ public:
+  virtual ~CSSTiming() {}
+
+  // TODO(csharrison): Also record update style time before first paint.
+  void recordAuthorStyleSheetParseTime(double us);
+
+  // In microseconds.

[Bryan McQuade, 2016/11/04 14:11:30]

iirc most of our other fields operate in seconds, rather than micros. if you are
using double, let's keep it consistent and also use seconds here. it'd be really
easy for someone who's used to perf timing code operating on seconds to
mistakenly misinterpret this as being in seconds, if they happen to miss the
comment.

[Charlie Harrison, 2016/11/04 16:05:19]

Done.

+  double authorStyleSheetParseDurationBeforeFCP() const {
+    return m_parseTimeBeforeFCP;
+  }
+
+  static CSSTiming& from(Document&);
+  DECLARE_VIRTUAL_TRACE();
+
+ private:
+  explicit CSSTiming(Document&);
+
+  // In microseconds.
+  double m_parseTimeBeforeFCP = 0;
+
+  Member<Document> m_document;
+  Member<PaintTiming> m_paintTiming;
+};
+
+}  // namespace blink
+
+#endif  // CSSTiming_h