backing out 4df2e13d4157 (bug 891993), to prevent build bustage from warnings in a 3rd-party .h file that'll be landing in Bug 837199.

toolkit/components/downloads/Makefile.in

@@ -12,7 +12,6 @@ include $(DEPTH)/config/autoconf.mk
 
 LIBRARY_NAME = download_s
 LIBXUL_LIBRARY = 1
-FAIL_ON_WARNINGS = 1
 
 include $(topsrcdir)/config/rules.mk