SkString: use c++11 vsnprintf in windows

Change-Id: I47337525714c4ee29bbc67c336d070b5cec2e57d
Reviewed-on: https://skia-review.googlesource.com/109484
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
1 file changed