Warnings are compiler version dependent (and even optimization level dependent, surprisingly). It looks like that one was introduced by gcc 4.6, so perhaps the

Warnings are compiler version dependent (and even optimization level dependent, surprisingly). It looks like that one was introduced by gcc 4.6, so perhaps the author of the patch was using a different version. Chrome builds with -Werror on by default but, because of the above reason, recommends any person attempting to build it to turn it off (and provides a flag to do as much).