Enable -Wshorten-64-to-32 for core/fpdfapi/cmaps

No code changes as it is already clean.

Change-Id: Ic741b2944a94562d7b7c423e0208d1f9ebdd3e70
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/89670
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fpdfapi/cmaps/BUILD.gn b/core/fpdfapi/cmaps/BUILD.gn
index 9a64298..8fbf83a 100644
--- a/core/fpdfapi/cmaps/BUILD.gn
+++ b/core/fpdfapi/cmaps/BUILD.gn
@@ -70,7 +70,11 @@
     "fpdf_cmaps.cpp",
     "fpdf_cmaps.h",
   ]
+  cflags = []
   configs += [ "../../../:pdfium_strict_config" ]
   deps = [ "../../fxcrt" ]
   visibility = [ "../../../*" ]
+  if (is_clang) {
+    cflags += [ "-Wshorten-64-to-32" ]
+  }
 }