Update code to include base/files/file_util.h

file_util.h was moved from base to base/files
(https://codereview.chromium.org/468253002). This CL updates platform2
code to include base/files/file_util.h instead of base/file_util.h.

BUG=chromium:411001
TEST=Trybot run on paladin, release, and chromiumos-sdk builders.

Change-Id: I488925b54615e131e508a460dc1a27f88168f936
Reviewed-on: https://chromium-review.googlesource.com/216851


Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>