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Landing this right after the M58 branch will complicate backports. I'm
going to land it right before the M59 branch.

This reverts commit a0c4422b.

Original description:
Now that mustash can use standard aura windows we don't need the ash/aura
vs. ash/mus vs. ash/common distinction. Eliminate //ash/common. Also move
files in //ash/common/system/chromeos out to //ash/system because ash only
compiles on chromeos these days.

Part 1 moves //ash/common/system into //ash/system. The CQ cannot handle
moving all files at once, see https://codereview.chromium.org/2734653002/

BUG=690096
TEST=compiles, ash_unittests
TBR=oshima@chromium.org

Review-Url: https://codereview.chromium.org/2732813002
Cr-Original-Commit-Position: refs/heads/master@{#454786}
Review-Url: https://codereview.chromium.org/2731873002 .
Cr-Commit-Position: refs/heads/master@{#454799}