Support for OpenType Font Variations on Windows

Enable support for variable fonts on Windows through using
SkFontMgr_Custom_Empty, a FreeType backed font-blob only SkTypeface
factory (no access to system fonts). We will use a hybrid DirectWrite
and FreeType font stack on Windows for at least as long as most of
Windows versions we support with Chrome still do not have native support
for font variations.

Thanks to Ben Wagner for the help with enabling and prototyping this.

BUG=700926

Review-Url: https://codereview.chromium.org/2780133002
Cr-Commit-Position: refs/heads/master@{#460895}
10 files changed