Restore fetch-gn instructions.

The automatic DEPS hook doesn't actually work with the checkout instructions we have on the site.

BUG=skia:6058

NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=6180

Change-Id: Ib1caa617791d3de2b60ddf00fed24d28fd9f83d1
Reviewed-on: https://skia-review.googlesource.com/6180
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed