Stop manually setting -Wshadow.

Use //build/config/compiler:noshadowing instead.

Change-Id: I9be2692daea967f57a49768206bd9a83da9b0e57
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/63090
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index ec0a94f..f27a2f4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -83,11 +83,9 @@
   configs = [
     ":pdfium_common_config",
     ":pdfium_public_config",
+    "//build/config/compiler:noshadowing",
   ]
   defines = []
-  if (is_clang) {
-    cflags += [ "-Wshadow" ]
-  }
   if (is_linux) {
     if (current_cpu == "x64") {
       defines += [ "_FX_CPU_=_FX_X64_" ]