RenderTextHarfBuzz: Implement font fallback for Win and Linux

ui/gfx/font_fallback.h

+// Copyright 2014 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 UI_GFX_FONT_FALLBACK_H_
+#define UI_GFX_FONT_FALLBACK_H_
+
+#include <string>
+#include <vector>
+
+namespace gfx {
+
+// Given a font family name, returns the names of font families that are
+// suitable for fallback.
+std::vector<std::string> GetFallbackFontFamilies(
+    const std::string& font_family);
+
+}  // namespace gfx
+
+#endif  // UI_GFX_FONT_FALLBACK_H_