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 3 the rest of //ash/common into //ash. The CQ cannot move all the
files at once, see https://codereview.chromium.org/2734653002/

BUG=690096
TEST=compiles, ash_unittests
TBR=oshima@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2729363002
Cr-Commit-Position: refs/heads/master@{#454793}