Add support for WGSL writing to BUILD.gn

BUG=tint:49

Change-Id: I4473176d4177a719b7b2659f765b6b467ac43c84
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/19682
Reviewed-by: dan sinclair <dsinclair@google.com>
diff --git a/tint_overrides_with_defaults.gni b/tint_overrides_with_defaults.gni
index f302f38..5732e2d 100644
--- a/tint_overrides_with_defaults.gni
+++ b/tint_overrides_with_defaults.gni
@@ -51,7 +51,10 @@
     tint_build_wgsl_reader = false
   }
 
-  # TODO(rharrison): Implement support for the reset of the reader/writers
+  # Build the WGSL output writer
+  if (!defined(tint_build_wgsl_writer)) {
+    tint_build_wgsl_writer = false
+  }
 
   # Generate fuzzers
   # TODO(rharrison): Implement fuzzer support