Use the default theme as the fallback UI in Blink

ui/native_theme/fallback_theme.h

Index: ui/native_theme/fallback_theme.h
diff --git a/ui/native_theme/fallback_theme.h b/ui/native_theme/fallback_theme.h
new file mode 100644
index 0000000000000000000000000000000000000000..2a699366f50fe563f53def9bc581cd67a961afd5
--- /dev/null
+++ b/ui/native_theme/fallback_theme.h
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+// Copyright (c) 2012 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_NATIVE_THEME_FALLBACK_THEME_H_
+#define UI_NATIVE_THEME_FALLBACK_THEME_H_
+
+#include "ui/native_theme/native_theme_base.h"
+
+namespace ui {
+
+// This theme can draw UI controls on every platform. This is only used when
+// zooming a web page and the native theme doesn't support scaling.
+class NATIVE_THEME_EXPORT FallbackTheme : public NativeThemeBase {
+ public:
+  FallbackTheme();
+  virtual ~FallbackTheme();
+
+ private:
+  // Overridden from NativeThemeBase:
+  virtual SkColor GetSystemColor(ColorId color_id) const OVERRIDE;
+
+  DISALLOW_COPY_AND_ASSIGN(FallbackTheme);
+};
+
+}  // namespace ui
+
+#endif  // UI_NATIVE_THEME_FALLBACK_THEME_H_