Enable precompiled headers for Blink on Windows.

third_party/WebKit/Source/core/win/Precompile-core.h

+// 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.
+
+/*

[Nico, 2016/11/21 15:42:25]

why c-style comment

[sof, 2016/11/21 19:24:58]

thanks, normalized them all.

+ * Precompiled header for WebKit when built on Windows.
+ *
+ * Using precompiled headers speeds the build up significantly.  On a
+ * fast machine (HP Z600, 12 GB of RAM), an ~18% decrease in full
+ * build time was measured with only the stdlib headers added. More
+ * with the Blink specific headers.
+ */
+
+#if defined(PrecompileCore_h_)
+#error You shouldn't include the precompiled header file more than once.
+#endif
+
+#define PrecompileCore_h_
+
+#include "build/win/Precompile.h"
+
+// In Blink a lot of operations center around dom and Document, or around
+// layout/rendering and LayoutObject. Those two headers are in turn pulling
+// in large parts of Blink's other headers which means that every compilation
+// unit is compiling large parts of Blink. By precompiling Document.h
+// and LayoutObject.h we only have to compile those parts once rather
+// than 1500 times. It can make a large difference in compilation
+// times (3-4 times faster).
+#include "core/dom/Document.h"
+#include "core/layout/LayoutObject.h"