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: I7aba1c7bb7dfe0598cdb077cb97def752b8bac79
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/232660
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35110}

BUILD.gn

@@ -305,7 +305,10 @@ config("common_config") {
     defines += [ "WEBRTC_EXCLUDE_AUDIO_PROCESSING_MODULE" ]
   }
 
-  cflags = []
+  # TODO(webrtc:13219): Fix -Wshadow instances and enable.
+  if (is_clang) {
+    cflags += [ "-Wno-shadow" ]
+  }
 
   if (build_with_chromium) {
     defines += [