Remove unnecessary codes (associated with save image feature)

This CL removes the unnecessary codes which is related to the |save image for
canvas| feature. We no longer need the codes because we have already introduced
the saveImageFromData() method to the following CL.
  https://crrev.com/518043003

BUG=401888

Review URL: https://codereview.chromium.org/530913003

git-svn-id: svn://svn.chromium.org/blink/trunk@182334 bbb929c8-8fbe-4397-9dbb-9b2b20218538