move pdfium skia build forward to match chrome

see https://codereview.chromium.org/2843823003/

skia is relying on vulkan in ToT so add that
to pdfium's BUILD.gn. Even if pdfium's skia
DEPS doesn't have third_pary/vulkan, no one
seems to mind that the include directory
doesn't exist.

R=dsinclair@chromium.org

Change-Id: I1363f158755caa5e45031c84c1687134096b3ba2
Reviewed-on: https://pdfium-review.googlesource.com/4570
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Cary Clark <caryclark@google.com>
diff --git a/skia/BUILD.gn b/skia/BUILD.gn
index d4a8371..0760246 100644
--- a/skia/BUILD.gn
+++ b/skia/BUILD.gn
@@ -33,6 +33,7 @@
     "//third_party/skia/include/pipe",
     "//third_party/skia/include/ports",
     "//third_party/skia/include/utils",
+    "//third_party/skia/third_party/vulkan",
   ]
 
   defines = []