platform2: Explicitly use base:: namespace with CommandLine

The current revision of libchrome is removing CommandLine class
from global namespace and only retain the one in base::.

Sweep the code to use the correct namespace in anticpation of
libchrome revision roll.

BUG=None
TEST=./build_packages

Change-Id: Ib7ca448dac1204c1e36ab053e775a5a5214997ac
Reviewed-on: https://chromium-review.googlesource.com/263785


Trybot-Ready: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>