www/chromium: fixup last commit

• Make __FreeBSD_versoin check actually work
• Implement OS::GetCurrnetThreadId on FreeBSD 8
• Move DragonFly's condtional to its own #elif for clarity
• Add NetBSD conditional for reference

Differential Revision: https://reviews.freebsd.org/D1857
Submitted by: jbeich
Reviewed by: marino, myself
MFH: 2015Q1