Force -Wno-shadow to avoid variable shadowing warnings.

Long term, ideally, these would be fixed and this flag can be removed.
For now, this is an expedient way to allow enabling -Wshadow in
Chromium.

Bug: chromium:794619
Change-Id: Idd3b6eaf61cc6e2a32eecf94d6cc6a41afb5f4eb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/64761
Auto-Submit: Peter Kasting <pkasting@google.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/common/BUILD.gn

@@ -94,6 +94,9 @@ config("dawn_internal") {
   visibility = [ "../*" ]
 
   cflags = []
+  if (is_clang) {
+    cflags += [ "-Wno-shadow" ]
+  }
 
   # Enable more warnings that were found when using Dawn in other projects.
   # Add them only when building in standalone because we control which clang