Enable precompiled headers for Blink on Windows.

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

Index: third_party/WebKit/Source/core/win/Precompile-core.h
diff --git a/third_party/WebKit/Source/core/win/Precompile-core.h b/third_party/WebKit/Source/core/win/Precompile-core.h
new file mode 100644
index 0000000000000000000000000000000000000000..53ba37a3b519065d0ec449061795a90670e4c3ca
--- /dev/null
+++ b/third_party/WebKit/Source/core/win/Precompile-core.h
@@ -0,0 +1,20 @@
+// 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.
+
+#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"