Fix compiler warning for MSVC

BUG=skia:7954

Change-Id: I2b100f723a88a57aaa4de08860f40ab73b9b674f
Reviewed-on: https://skia-review.googlesource.com/128580
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed