Enable warnings for shadowed variables

This CL adds in -Wshadow that will throw a warning whenever a variable
is being shadowed.

BUG=pdfium:1137

Change-Id: I879683fbf2f84c6609cf496308ddadb74c2f26aa
Reviewed-on: https://pdfium-review.googlesource.com/41250
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index b3a56f7..99e71cf 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -72,6 +72,9 @@
   cflags = []
   configs = [ ":pdfium_common_config" ]
   defines = [ "V8_DEPRECATION_WARNINGS" ]
+  if (is_clang) {
+    cflags += [ "-Wshadow" ]
+  }
   if (is_linux) {
     if (current_cpu == "x64") {
       defines += [ "_FX_CPU_=_FX_X64_" ]