Flesh out PowerManagerMojo::ScreenBrightnessChanged

Type mapping is added for the change cause enum and the
BacklightBrightnessChange struct

Bug: 644348
Change-Id: I8e194790324b2e71b5690c7bf6ebc72ea09a4db8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1570250
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653685}
11 files changed