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Convert icons and images to use the newly-stabilized ImageSkia mojo
transport. This allows high DPI images. It is also higher performance
than the existing crosapi::Bitmap serializer because it uses shared
memory for large images.

Lacros still sends icons and images in the old format to allow
backwards compatibility with older ash. We can remove this old
serialization in 6 weeks or so. Until then we won't see the
performance benefit.

Manually tested on gLinux with high-DPI screen.

Bug: 1113889
Test: added to unit_tests
Change-Id: I1cf4b86512fd59e287a32dfb4ac2d59d2962a6f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2388152Reviewed-by: Jun Mukai <mukai@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803711}