Completes sniffing for mp4 signature according
to the spec at:
  https://mimesniff.spec.whatwg.org/#signature-for-mp4
Clause 6.2.1

Fixes #8773

Change-Id: Icfc4a23324ae249db52c94a21c0e8509e1833e19
Reviewed-on: https://go-review.googlesource.com/16951Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>