Replace image_messages.h with Mojo service

      Implement ImageDownloader service and register it into RenderFrame
ServiceRegistry. This Mojo service will do the job did by
ImageLoadingHelper before.

Re-use mojom files:
third_party/mojo_services/src/geometry/public/interfaces/
geometry.mojom
skia/public/interfaces/bitmap.mojom

TBR=rockot@chromium.org,amistry@google.com,darin@chromium.org,nasko@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1085783002

Cr-Commit-Position: refs/heads/master@{#337370}