Also implement it without reference to crypto/asn1 or fake ASN1_INTEGERs and
add a test. Some platform crypto APIs only give back the key size, and not the
encoded signature length. No sense in implementing it twice.

BUG=347404,499653

Change-Id: I9aa27d52674375f8b036e57bb5850f091c9b25dd
Reviewed-on: https://boringssl-review.googlesource.com/5080
Reviewed-by: Adam Langley <agl@google.com>