Font variations.

Multiple Master and TrueType fonts support variation axes.
This implements back-end support for axes on platforms which
support it.

Committed: https://skia.googlesource.com/skia/+/05773ed30920c0214d1433c07cf6360a05476c97

Committed: https://skia.googlesource.com/skia/+/3489ee0f4fa34f124f9de090d12bdc2107d52aa9

Review URL: https://codereview.chromium.org/1027373002
24 files changed