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The structure is just a representation of a bitmap. I'd like to use it
in another API for an icon.

* Extract the mojom struct into its own file
* Rename the C++ class and mojo traits serialization helpers
* Rename the |bitmap| member to |pixels| so we don't have to type
  bitmap.bitmap

None of the implementation changed. Gerrit shows file add/deletes
because the renames touched most lines.

Bug: 1113889
Change-Id: Ie0d9aa30d913089adab52d0f4bd3ac050575e96c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352645Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797510}