[Extensions UI] Add a placeholder icon for extensions in the toolbar

extensions/browser/extension_icon_placeholder.h

Index: extensions/browser/extension_icon_placeholder.h
diff --git a/extensions/browser/extension_icon_placeholder.h b/extensions/browser/extension_icon_placeholder.h
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index 0000000000000000000000000000000000000000..5c5e90238f97854033a45121160078972cfe7805
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+++ b/extensions/browser/extension_icon_placeholder.h
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+// Copyright 2015 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 EXTENSIONS_BROWSER_EXTENSION_ICON_PLACEHOLDER_H_
+#define EXTENSIONS_BROWSER_EXTENSION_ICON_PLACEHOLDER_H_
+
+#include <string>
+
+#include "base/macros.h"
+#include "base/strings/string16.h"
+#include "extensions/common/constants.h"
+#include "ui/gfx/image/canvas_image_source.h"
+#include "ui/gfx/image/image.h"
+
+namespace gfx {
+class Canvas;
+class Size;
+}
+
+namespace extensions {
+
+// An extension icon image with a gray background and the first letter of the
+// extension name, so that not all extensions without an icon look the same.
+class ExtensionIconPlaceholder : public gfx::CanvasImageSource {
+ public:
+  ExtensionIconPlaceholder(extension_misc::ExtensionIcons size,
+                           const std::string& name);
+  ~ExtensionIconPlaceholder() override;
+
+  // Creates an image backed by an ImageSkia with the ExtensionIconPlaceholder
+  // as its image source.
+  static gfx::Image CreateImage(extension_misc::ExtensionIcons size,
+                                const std::string& name);
+
+ private:
+  // gfx::CanvasImageSource:
+  void Draw(gfx::Canvas* canvas) override;
+
+  // The size this placeholder is filling.
+  extension_misc::ExtensionIcons icon_size_;
+
+  // The first letter of the extension's name.
+  base::string16 letter_;
+
+  // The gray background image, on top of which the letter is drawn.
+  gfx::Image base_image_;
+
+  DISALLOW_COPY_AND_ASSIGN(ExtensionIconPlaceholder);
+};
+
+}  // namespace extensions
+
+#endif  // EXTENSIONS_BROWSER_EXTENSION_ICON_PLACEHOLDER_H_