update obsolete code.google.com documentation links

This is a documentation-only change.

Disabling presubmit checks, due to "noparent" settings for the following files:
components/policy/resources/policy_templates.json
content/common/font_config_ipc_linux.h

BUG=567488
NOPRESUBMIT=true
TBR=atwilson,dcheng

Review URL: https://codereview.chromium.org/1592403002

Cr-Original-Commit-Position: refs/heads/master@{#374213}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: df175a8c3d1c1060f96d123a5896fdfbe4b3782d

tools/gn/misc/emacs/gn-mode.el

diff --git a/tools/gn/misc/emacs/gn-mode.el b/tools/gn/misc/emacs/gn-mode.el
index 3cb7341..6682290 100644
--- a/tools/gn/misc/emacs/gn-mode.el
+++ b/tools/gn/misc/emacs/gn-mode.el
@@ -14,7 +14,7 @@
 
 ;; A major mode for editing GN files. GN stands for Generate Ninja. GN is the
 ;; meta build system used in Chromium. For more information on GN, see the GN
-;; manual: <https://code.google.com/p/chromium/wiki/gn>
+;; manual: <https://chromium.googlesource.com/chromium/src/+/master/tools/gn/README.md>
 
 ;;; To Do: