add platform_font_manager()

This string replaces {major_}platform_os_name() and
platform_extra_config() for the font-focused GMs using them.

The GDI bots will report "GDI", the NativeFont bots their OS,
and other bots "".

Change-Id: I8f7bb1ffea3cc91601c98b4ccff7a3a234ac77d3
Reviewed-on: https://skia-review.googlesource.com/70500
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
9 files changed