Use atomic-file-writing primitive in libchrome

Instead of implementing my own atomic file writing, use the one
that's in libchrome.

BUG=None
TEST=unit tests
CQ-DEPEND=CL:191154

Change-Id: I9e6d16dce63e88dfb62483536968a293d7f8270f
Reviewed-on: https://chromium-review.googlesource.com/191134
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Queue: Chris Masone <cmasone@chromium.org>