Enable -Wshorten-64-to-32 for core/fdrm.

This code in this directory is already free of such conversions.

Change-Id: I25756e97ce4a6ff70e4b4581ee56bc9296fe2a80
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/88570
Auto-Submit: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fdrm/BUILD.gn

diff --git a/core/fdrm/BUILD.gn b/core/fdrm/BUILD.gn
index 8e79c14..0b14d7f 100644
--- a/core/fdrm/BUILD.gn
+++ b/core/fdrm/BUILD.gn
@@ -12,9 +12,13 @@
     "fx_crypt_aes.cpp",
     "fx_crypt_sha.cpp",
   ]
+  cflags = []
   configs += [ "../../:pdfium_strict_config" ]
   deps = [ "../fxcrt" ]
   visibility = [ "../../*" ]
+  if (is_clang) {
+    cflags += [ "-Wshorten-64-to-32" ]
+  }
 }
 
 pdfium_unittest_source_set("unittests") {