Use just one style for the shim.

It's currently a mix of GoogleCPlusPlusStyle and unix_hacker_style. Since it's
now been thoroughly C++-ified, let's go with the former. This also matches the
tool, our other bit of C++ code.

Change-Id: Ie90a166006aae3b8f41628dbb35fcd64e99205df
Reviewed-on: https://boringssl-review.googlesource.com/3348
Reviewed-by: Adam Langley <agl@google.com>