The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4b282d1865ab5c2273f7cb03be89eb0e55a58768

commit 4b282d1865ab5c2273f7cb03be89eb0e55a58768
Author: injae <injae@google.com>
Date: Thu Jun 29 00:26:05 2017

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

JPEG Codec does not support transparent pixels, so PNG Codec was added.

Added a new parameter to chrome_render_frame_observer#RequestThumbnailForContextNode which controls the codec used to encode the image.

Kept current implementations of RequestThumbnailForContextNode using JPEG Codec.
Currently, the only use for PNG Codec is for the context menu

BUG= 730327 

Review-Url: https://codereview.chromium.org/2943363003
Cr-Commit-Position: refs/heads/master@{#483219}

[modify] https://crrev.com/4b282d1865ab5c2273f7cb03be89eb0e55a58768/chrome/browser/renderer_context_menu/render_view_context_menu_browsertest.cc
[modify] https://crrev.com/4b282d1865ab5c2273f7cb03be89eb0e55a58768/chrome/browser/ui/android/context_menu_helper.cc
[modify] https://crrev.com/4b282d1865ab5c2273f7cb03be89eb0e55a58768/chrome/browser/ui/tab_contents/core_tab_helper.cc
[modify] https://crrev.com/4b282d1865ab5c2273f7cb03be89eb0e55a58768/chrome/common/thumbnail_capturer.mojom
[modify] https://crrev.com/4b282d1865ab5c2273f7cb03be89eb0e55a58768/chrome/renderer/chrome_render_frame_observer.cc
[modify] https://crrev.com/4b282d1865ab5c2273f7cb03be89eb0e55a58768/chrome/renderer/chrome_render_frame_observer.h