Update skia to use ifdefs for Vulkan code instead of dummy header

Bug: skia:6721
Change-Id: I80a4c9f2acc09c174497f625c50ed12a8bb76505
Reviewed-on: https://skia-review.googlesource.com/19547
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
8 files changed