Added ASSERT_SIZE macro for checking size of size-sensitive classes

third_party/WebKit/Source/wtf/SizeAssertions.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.
+
+#ifndef WTF_SizeAssertions_h
+#define WTF_SizeAssertions_h
+
+namespace WTF {
+
+// The ASSERT_SIZE macro can be used to check that a given struct is the same
+// size as a class. This is useful to visualize where the space is being used in
+// a class, as well as give a useful compile error message when the size doesn't
+// match the expected value.
+template<class T, class U> struct assert_size {
+    template<int ActualSize, int ExpectedSize> struct assertSizeEqual {
+        static_assert(ActualSize == ExpectedSize, "Class should stay small");
+        static const bool innerValue = true;
+    };
+    static const bool value = assertSizeEqual<sizeof(T), sizeof(U)>::innerValue;
+};
+
+} // namespace WTF
+
+#define ASSERT_SIZE(className, sameSizeAsClassName) \
+    static_assert(WTF::assert_size<className, sameSizeAsClassName>::value, "");
+
+#endif // WTF_SizeAssertions_h