Set a default value for dawn_glm_dir like other GN build overrides

Bug: dawn:437
Change-Id: Iffdb281df32a506b1cf21823d27ee18710427457
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/22102
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jiajie Hu <jiajie.hu@intel.com>

scripts/dawn_overrides_with_defaults.gni

@@ -41,6 +41,10 @@ if (!defined(dawn_glfw_dir)) {
   dawn_glfw_dir = "//third_party/glfw"
 }
 
+if (!defined(dawn_glm_dir)) {
+  dawn_glm_dir = "//third_party/glm"
+}
+
 if (!defined(dawn_googletest_dir)) {
   dawn_googletest_dir = "//third_party/googletest"
 }