[build] Fix erroneously removed warning flag.

Bug: chromium:709420
TBR=marja@chromium.org

Change-Id: I71cf02137bcc7d8e163162c6dc1be6b906422281
Reviewed-on: https://chromium-review.googlesource.com/474747Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44552}

BUILD.gn

@@ -458,10 +458,10 @@ config("toolchain") {
       # TODO(hans): Remove once http://crbug.com/428099 is resolved.
       "-Winconsistent-missing-override",
     ]
-    #if (v8_current_cpu == "x64" || v8_current_cpu == "arm64" ||
-    #    v8_current_cpu == "mips64el") {
-    #  cflags += [ "-Wshorten-64-to-32" ]
-    #}
+    if (v8_current_cpu == "x64" || v8_current_cpu == "arm64" ||
+        v8_current_cpu == "mips64el") {
+      cflags += [ "-Wshorten-64-to-32" ]
+    }
   }
 
   if (is_win) {