Introduce ForwardsTextBuffer and BackwardsTextBuffer

third_party/WebKit/Source/core/editing/iterators/BackwardsTextBuffer.h

Index: third_party/WebKit/Source/core/editing/iterators/BackwardsTextBuffer.h
diff --git a/third_party/WebKit/Source/core/editing/iterators/BackwardsTextBuffer.h b/third_party/WebKit/Source/core/editing/iterators/BackwardsTextBuffer.h
new file mode 100644
index 0000000000000000000000000000000000000000..bbc2a12cec0ef22b96a9ae5d0fbd37d5fa9cd4ec
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+++ b/third_party/WebKit/Source/core/editing/iterators/BackwardsTextBuffer.h
@@ -0,0 +1,26 @@
+// 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 BackwardsTextBuffer_h
+#define BackwardsTextBuffer_h
+
+#include "core/editing/iterators/TextBufferBase.h"
+
+namespace blink {
+
+class CORE_EXPORT BackwardsTextBuffer final : public TextBufferBase {
+    STACK_ALLOCATED();
+    WTF_MAKE_NONCOPYABLE(BackwardsTextBuffer);
+public:
+    BackwardsTextBuffer() {}
+    const UChar* data() const override;
+
+private:
+    UChar* calcDestination(size_t length) override;
+    void shiftData(size_t oldCapacity) override;
+};
+
+} // namespace blink
+
+#endif // TextBuffer_h