message: Set a default value for dawn_glm_dir like other GN build overrides
diff: @@ -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"
  }
  