Decode system font weight correctly on Mac.

The -1 to 1 weights reported by CTFontDescriptors have different mappings
depending on if the CTFont is native or created from a CGDataProvider.

Change-Id: I725b04a31c224a4b01c0763137fa04bb4088def7
Reviewed-on: https://skia-review.googlesource.com/6979
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed