Add ANGLE vulkan GN to gypi whitelist.

ANGLE still uses GYP for a standalone build. Until we switch to full
GN (not currently being worked on) we need this script for all of our
build files.

BUG=605987

Review-Url: https://codereview.chromium.org/2320743002
Cr-Commit-Position: refs/heads/master@{#417374}

.gn

@@ -260,6 +260,7 @@ exec_script_whitelist = [
   "//third_party/android_platform/BUILD.gn",
   "//third_party/angle/BUILD.gn",
   "//third_party/angle/src/tests/BUILD.gn",
+  "//third_party/angle/src/vulkan_support/BUILD.gn",
   "//third_party/boringssl/BUILD.gn",
   "//third_party/catapult/tracing/BUILD.gn",
   "//third_party/cld_2/BUILD.gn",