Share precompiled header setup for blink between Windows and Mac.

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

Index: third_party/WebKit/Source/core/mac/Precompile-core.h
diff --git a/third_party/WebKit/Source/core/mac/Precompile-core.h b/third_party/WebKit/Source/core/mac/Precompile-core.h
new file mode 100644
index 0000000000000000000000000000000000000000..f87a6a62cdf164b0a209e009ff861dbe8b942e02
--- /dev/null
+++ b/third_party/WebKit/Source/core/mac/Precompile-core.h
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+// Copyright 2017 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/mac/Prefix.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"