Enable -Werror for LLVM build
Enable -Werror for LLVM build

We're often added warnings to our build without realizing it, having -Werror will catch these.

R=dschuff@chromium.org
TEST= git cl try

Review URL: https://codereview.chromium.org/863093002

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client