Condense namespaces in tint/transform.

This PR condenses the namespaces in the tint/transform folder.

Change-Id: Idf448870ccf90f892b9186f7aab7bb0ac9deda17
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86029
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
diff --git a/src/tint/transform/builtin_polyfill_test.cc b/src/tint/transform/builtin_polyfill_test.cc
index 49e0471..c5cc2c5 100644
--- a/src/tint/transform/builtin_polyfill_test.cc
+++ b/src/tint/transform/builtin_polyfill_test.cc
@@ -18,8 +18,7 @@
 
 #include "src/tint/transform/test_helper.h"
 
-namespace tint {
-namespace transform {
+namespace tint::transform {
 namespace {
 
 using Level = BuiltinPolyfill::Level;
@@ -1117,5 +1116,4 @@
 }
 
 }  // namespace
-}  // namespace transform
-}  // namespace tint
+}  // namespace tint::transform