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>

scripts/dawn_features.gni

@@ -59,7 +59,7 @@ declare_args() {
   # if performing reflection on systems that the platform language
   # shader is SPIR-V, since there isn't an instance of the
   # GLSL/HLSL/MSL compiler. This implicitly pulls in the GLSL
-  # compiler, since it is a sub-class of if.
+  # compiler, since it is a sub-class of it.
   dawn_enable_cross_reflection = false
 
   # Enables error injection for faking failures to native API calls
@@ -68,6 +68,10 @@ declare_args() {
 
   # Whether Dawn should enable X11 support.
   dawn_use_x11 = is_linux && !is_chromeos
+
+  # Enable support WGSL for shaders. Currently this is
+  # experimental, and requires dawn_tint_dir to be set.
+  dawn_enable_wgsl = false
 }
 
 # GN does not allow reading a variable defined in the same declare_args().

scripts/dawn_overrides_with_defaults.gni

@@ -71,3 +71,8 @@ if (!defined(dawn_vulkan_validation_layers_dir)) {
   # Default to VVLs not being available.
   dawn_vulkan_validation_layers_dir = ""
 }
+
+if (!defined(dawn_tint_dir)) {
+  # Default to Tint not being available.
+  dawn_tint_dir = ""
+}