Add Win Clang Vulkan builds.

Fix Clang compiler warning.

Change-Id: Ic6f5b161058d1ed2df45e12b72dbff3b486ec986
Reviewed-on: https://skia-review.googlesource.com/56740
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
3 files changed