build: enable -Wshadow

which prevents shadowing variables.

BUG=webrtc:42223409

Change-Id: Icfaa101dc25717a89d1ac50d446dd9b69e18764c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/387100
Reviewed-by: Christoffer Dewerin <jansson@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#44563}

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 2d84f3c..4ad7f3e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -377,8 +377,7 @@
 
   if (is_clang) {
     cflags += [
-      # TODO(webrtc:13219): Fix -Wshadow instances and enable.
-      "-Wno-shadow",
+      "-Wshadow",
 
       # See https://reviews.llvm.org/D56731 for details about this
       # warning.