gn-mode: Clear indent-tabs-mode

GN does not allow tabs, so the mode should always set
indent-tabs-mode to nil (off), regardless of the user's
global setting.

R=erg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1341943002

Cr-Commit-Position: refs/heads/master@{#348753}

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

@@ -132,6 +132,7 @@ variable name or the '{{' and '}}' which surround it."
   (setq-local comment-use-syntax t)
   (setq-local comment-start "#")
   (setq-local comment-end "")
+  (setq-local indent-tabs-mode nil)
 
   (setq-local fill-paragraph-function 'gn-fill-paragraph)