Removed mentions of obsolete flag

We've removed the service worker specific flag in Chrome 38 (current
canary). One only need the experimental web platform feature flag.

Notes:
1. Not entirely sure about Opera's case but I assume that it's similar
2. Technically non Canary Chrome still need the SW specific flag but
that's perhaps too much details / could be added as one footnote.
Merge pull request #4 from KenjiBaheux/master

Removed mentions of obsolete flag