Adding support for testing BUILD.gn

BUG=tint:50

Change-Id: Ie19a1af6a97a49a67dcc7474991c960b986ddebe
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/19782
Reviewed-by: David Neto <dneto@google.com>
diff --git a/tint_overrides_with_defaults.gni b/tint_overrides_with_defaults.gni
index 5732e2d..a8861ea 100644
--- a/tint_overrides_with_defaults.gni
+++ b/tint_overrides_with_defaults.gni
@@ -22,6 +22,11 @@
     tint_spirv_tools_dir = "//third_party/spirv-tools"
   }
 
+  # Path to googletest checkout
+  if (!defined(tint_googletest_dir)) {
+    tint_googletest_dir = "//third_party/googletest"
+  }
+
   # Path to spirv-tools generated files, relative to |target_gen_dir|
   if (!defined(tint_spirv_tools_gen)) {
     tint_spirv_tools_gen = "third_party/spirv-tools"