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Renderer side paint preview implementation. This CL creates a mojo
service on a RenderFrameObserver that facilitates most of the paint
preview capture management.

Flow:
- Browser calls this service to request paint preview capture of current
  frame (future CL 1)
- PaintPreviewServiceImpl sets up for capture in Blink (this CL)
- Blink handles the capture via a flow similar to printing (future CL 2)
  - This will include invoking a request to the browser to forward
    requests to OOP subframes.
- PaintPreviewServiceImpl does post-processing and marshals a response
  (this CL)
  - Two artifacts are produced:
    - A SkPicture is written to the file handle provided by the browser
    - A proto containing metadata for the frame is sent via shared
      memory to the browser.
- Browser does some minor modifications to the proto then either stores
  the proto to disk or forwards the proto and SkPicture files to a
  utility process for playback. The browser will retain the links from
  the proto for hit testing. (future CLs)

Part of landing:
https://chromium-review.googlesource.com/c/chromium/src/+/1786583

Internal Doc: go/fdt-design

Bug: 994833
Change-Id: I88ffb89d92dfcb78b61a91a21df2b842c713f8b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824669Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706443}