Add vsync toggle for windows viewer.

Change-Id: I1a223c96fbffc24a3a9263304768c2d7946a7a9e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/204260
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed