Fixing transparent pixels appearing black when rendered for the context menu.

chrome/common/thumbnail_capturer.mojom

 // Copyright 2017 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.
 
 module chrome.mojom;
 
 import "ui/gfx/geometry/mojo/geometry.mojom";
 
+enum ImageFormat {
+  JPEG,
+  PNG,
+};
+
 interface ThumbnailCapturer {
-  // Requests a JPEG encoded thumbnail of the image selected by the most
-  // recently opened context menu. If no image is selected or there's an error
-  // capturing a thumbnail, |thumbnail_data| will be empty. If the image area is
-  // larger than |thumbnail_min_area_pixels| it will be downscaled to fit within
+  // Requests an encoded thumbnail of the image selected by the most recently
+  // opened context menu. The encoding format is specified as a parameter. If
+  // no image is selected or there's an error capturing a thumbnail,
+  // |thumbnail_data| will be empty. If the image area is larger than
+  // |thumbnail_min_area_pixels| it will be downscaled to fit within 
   // |thumbnail_max_size_pixels|.
   RequestThumbnailForContextNode(int32 thumbnail_min_area_pixels,
-                                 gfx.mojom.Size thumbnail_max_size_pixels)
+                                 gfx.mojom.Size thumbnail_max_size_pixels,
+                                 ImageFormat image_format)
       => (array<uint8> thumbnail_data, gfx.mojom.Size original_size);
 };