Use spvc's rules for spirv-cross

Switch over to using the rule for building spirv-cross from spvc. This
allows GN to avoid double symbol issues in future patches.

Roll third_party/shaderc/ 1d6155d86..30f0559dd (2 commits)

https://chromium.googlesource.com/external/github.com/google/shaderc/+log/1d6155d8679a..30f0559dd278

$ git log 1d6155d86..30f0559dd --date=short --no-merges --format='%ad %ae %s'
2019-11-27 rharrison Remove spvc's dependency on shaderc's util library (#914)
2019-11-27 rharrison Clean up how include dirs are propegated for spirv_cross (#912)

Created with:
  roll-dep third_party/shaderc

BUG=dawn:281

Change-Id: I49cf237dcab884e84180f83cbbb21acfa549fe61
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/14121
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 68e63d1..5081a4c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -134,8 +134,8 @@
     ":libdawn_native_headers",
     ":libdawn_native_utils_gen",
     "${dawn_root}/src/common",
+    "${dawn_shaderc_dir}:spirv_cross",
     "${dawn_spirv_tools_dir}:spvtools_val",
-    "third_party:spirv_cross",
   ]
   defines = []
   libs = []