Use C++11 inline initialization.

Google C++ style allows these. It's also considerably less tedious and
error-prone than defining an out-of-line constructor.

Change-Id: Ib76ccf6079be383722433046ac5c5d796dd1f525
Reviewed-on: https://boringssl-review.googlesource.com/4111
Reviewed-by: Adam Langley <agl@google.com>