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
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index 0000000000000000000000000000000000000000..8bb4551dce3fbded79fe64b8ed03653e10fb4cfa
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+// Copyright 2015 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.
+
+/*
+ * Precompiled header for WebKit when built on Windows using
+ * GYP-generated project files.  Not used by other build
+ * configurations.
+ *
+ * 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.
+// config.h contains some base macros that headers can expect are present.
+#include "core/dom/Document.h"
+#include "core/layout/LayoutObject.h"