cros: Split ShellPortMash into Mus and Mash subclasses

ShellPortMash is a mix of --mus and --mash support code.
Split it into two parts to make it easier to reason about
which code runs in which mode.

[email protected]

Bug: 779186
Test: ash_unittests, chrome --mus and chrome --mash
Change-Id: Ia6bfaf8e30826855d994e6604c244d68ffe086f5
Reviewed-on: https://chromium-review.googlesource.com/742130
Reviewed-by: James Cook <[email protected]>
Reviewed-by: Elliot Glaysher <[email protected]>
Commit-Queue: James Cook <[email protected]>
Cr-Commit-Position: refs/heads/master@{#512360}
15 files changed