Add Tint to BUILD.gn

Tint is behind a build flag, since Tint is not yet available in
chromium's third_party.

BUG=dawn:403

Change-Id: If5fa36b36216ef6965341b886c3bd70d27daf23a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/21300
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>

src/dawn_native/BUILD.gn

@@ -569,6 +569,11 @@ source_set("dawn_native_sources") {
       ]
     }
   }
+
+  if (dawn_enable_wgsl) {
+    deps += [ "${dawn_tint_dir}:libtint" ]
+    defines += [ "DAWN_ENABLE_WGSL" ]
+  }
 }
 
 # The static and shared libraries for dawn_native. Most of the files are