Improve variation comment for iOS.

CTFontCopyVariationAxes returns nullptr for fonts which began life with
CGFontCreateWithDataProvider on macOS 10.10 and earlier and iOS 9 and
earlier. The issue was fixed in maxOS 10.11 and iOS 10. Improve the
existing comments to make this clear.

Change-Id: I0de1734e266d30aab7d38d0dd752f66259f677d9
Reviewed-on: https://skia-review.googlesource.com/14278
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed