Initial support for using BoringSSL with NSS certificates.
This switches the USE_OPENSSL Linux and CrOS builds to continue
setting USE_NSS_CERTS. This lets it use BoringSSL for the crypto
and SSL stack and NSS for certificate verification. See design
doc for details on the flags:
https://docs.google.com/document/d/1x4DOCKwFkAxl9MGfd6snIzFigO4ku6Shuci0r5BzasQ/edit
On Linux, only client auth and OCSP stapling are missing. On ChromeOS,
there are some problematic USE_NSS_CERTS APIs in RSAPrivateKey to
resolve. We also still need to resolve crbug.com/347404 to maintain
parity.
As a follow-up, USE_OPENSSL_CERTS on non-Android can now be
removed (it was never supported anyway).
BUG=462040
Review URL: https://codereview.chromium.org/
881213004
Cr-Commit-Position: refs/heads/master@{#326222}