Use spirv-headers' BUILD.gn (#3148)

* Roll external/spirv-headers/ af64a9e82..dc77030ac (4 commits)

$ git log af64a9e82..dc77030ac --date=short --no-merges --format='%ad %ae %s'
2020-01-20 dneto Fix the license to match LICENSE
2020-01-20 syoussefi Add BUILD.gn
2019-11-20 Tobias.Hector Off-by-one errors
2019-11-20 Tobias.Hector Reserve a new block of 64 opcodes

Created with:
  roll-dep external/spirv-headers

* Use BUILD.gn from spirv-headers
diff --git a/BUILD.gn b/BUILD.gn
index 5ed85d9..eea3985 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -324,14 +324,6 @@
   }
 }
 
-source_set("spv_headers") {
-  sources = [
-    "$spirv_headers/include/spirv/1.2/GLSL.std.450.h",
-    "$spirv_headers/include/spirv/unified1/OpenCL.std.h",
-    "$spirv_headers/include/spirv/unified1/spirv.h",
-  ]
-}
-
 source_set("spvtools_headers") {
   sources = [
     "include/spirv-tools/instrument.hpp",
@@ -424,9 +416,9 @@
   ]
 
   public_deps = [
-    ":spv_headers",
     ":spvtools_core_enums_unified1",
     ":spvtools_headers",
+    "${spirv_headers}:spv_headers",
   ]
 
   if (build_with_chromium) {