Chromium's toolchains may now assume C++11 library support, so we may freely
use C++11 features. (Chromium's still in the process of deciding what to allow,
but we use Google's style guide directly, toolchain limitations aside.)

Change-Id: I1c7feb92b7f5f51d9091a4c686649fb574ac138d
Reviewed-on: https://boringssl-review.googlesource.com/6465
Reviewed-by: Adam Langley <agl@google.com>