Add in flag for using spvc instead of direct spriv-cross access

Currently does nothing interesting, future CLs will flesh out the
functionality.

BUG=dawn:281

Change-Id: I89750a45ff5a42a13e1494cafd433bb7ef719b10
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/13841
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index ca6a8c0..6079642 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -127,6 +127,11 @@
     "${dawn_spirv_tools_dir}:spvtools_val",
     "third_party:spirv_cross",
   ]
+  defines = []
+  if (dawn_use_spvc) {
+    deps += [ "${dawn_shaderc_dir}:libshaderc_spvc" ]
+    defines += [ "DAWN_USE_SPVC" ]
+  }
 
   configs += [ ":libdawn_native_internal" ]
   libs = []
@@ -500,7 +505,7 @@
       ]
     }
     if (dawn_enable_vulkan_validation_layers) {
-      defines = [
+      defines += [
         "DAWN_ENABLE_VULKAN_VALIDATION_LAYERS",
         "DAWN_VK_DATA_DIR=\"$vulkan_data_subdir\"",
       ]