libchromeos: Support building with libchrome r242728.

Also delete unused platform.* files.

BUG=chromium:331128,chromium:216672
TEST=built it
CQ-DEPEND=Ib6c567d9f92b5e24cba04d9c1bd57517ee0ee0d8
CQ-DEPEND=I6137e995c5642076d57f8b1cf8f3e587d2ee9c26

Change-Id: Ie6a27189b2880dbf2f7a172ded68aadbf75ab641
Reviewed-on: https://chromium-review.googlesource.com/181965
Reviewed-by: Daniel Erat <derat@chromium.org>
Tested-by: Daniel Erat <derat@chromium.org>
Commit-Queue: Daniel Erat <derat@chromium.org>
20 files changed