Run bin/fetch-gn as a gclient hook.

Seems like the bots don't run hooks.  That's okay, even ideal.
They'll keep getting GN via recipes.

DOCS_PREVIEW= https://skia.org/?cl=5725

Change-Id: I000bad3390dddaeb4548972f29c96b8b3288ea6c
Reviewed-on: https://skia-review.googlesource.com/5725
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Eric Boren <borenet@google.com>

DEPS

@@ -45,3 +45,10 @@ deps = {
 }
 
 recursedeps = [ "common" ]
+
+hooks = []
+
+import os
+import sys
+if os.path.exists('bin/fetch-gn'):
+  hooks.append({ 'action': [sys.executable, 'bin/fetch-gn'] })

site/user/build.md

@@ -14,10 +14,6 @@ them.
 Quickstart
 ----------
 
-After gclient sync, run `fetch-gn` to make sure you have GN.
-
-    gclient sync && python bin/fetch-gn
-
 Run GN to generate your build files.
 
     gn gen out/Static --args='is_official_build=true'