[tint][glsl] Put GLSL validation behind a build flag.

This can be disabled if the outer project does not have glslang.

Change-Id: Ia0dad6e1ef0b9936bf035f94c9a04e33f62db6ce
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/155721
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/scripts/tint_overrides_with_defaults.gni b/scripts/tint_overrides_with_defaults.gni
index 54585bf..2daf496 100644
--- a/scripts/tint_overrides_with_defaults.gni
+++ b/scripts/tint_overrides_with_defaults.gni
@@ -95,6 +95,11 @@
     tint_build_glsl_writer = true
   }
 
+  # Build the GLSL output validator
+  if (!defined(tint_build_glsl_validator)) {
+    tint_build_glsl_validator = true
+  }
+
   # Build the Syntax Tree writer
   if (!defined(tint_build_syntax_tree_writer)) {
     tint_build_syntax_tree_writer = false