metrics: Replace 'OVERRIDE' with 'override' and fix linter

Replace OVERRIDE macro with native C++ 'override' keyword.
Also fixed the style issue of using both 'override' and 'virtual'
on member function overrides. Only one is required and cpplint
complains if both are specified now.

And since gobi-chromo-plugin is the only project that uses
libmetrics and is not C++11 yet, enabled C++11 features
in its makefile to allow for the 'override' keywords to
compile in the included metrics/metrics_library.h header.

BUG=None
TEST=USE="cellular buffet" ./build_packages

Change-Id: I28dace6dc4bcb07386632eaed890ce41564e8144
Reviewed-on: https://chromium-review.googlesource.com/212494


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