bin/sync : call tools/git-sync-deps now

Also, update some docs.

NOTRY=true
Change-Id: I7ad3375fc1cbf8f71ed42a460ecfe29ef6c1d85e
Reviewed-on: https://skia-review.googlesource.com/7657
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
3 files changed