Documentation: Consistancy

  * Don't rely on '#!', since it doesn't work for Windows cmd shell.
  * Consistantly use `tools/git-sync-deps` rather than `bin/sync`.
  * Always call `bin/gn` ranther than `gn` in case depot_tools is
    missing from the path.

NOTRY=true
Change-Id: I27909f2610d1bb3241874399d7d2f7286f99f43b
Reviewed-on: https://skia-review.googlesource.com/9640
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
6 files changed