gn: Check GN_EDITOR before EDITOR in environment variables.

gn: Check GN_EDITOR before EDITOR in environment variables.

It's a convention in unix for programs to first search for a program
specific editor variable. For example, SVN_EDITOR or GIT_EDITOR. This
makes GN follow this same convention.

BUG=none

Committed: https://crrev.com/a053ef69be56c9d073a8e383491e0f0854240250
Cr-Commit-Position: refs/heads/master@{#299798}

[Elliot Glaysher, 2014-10-15 21:09:49]

erg@chromium.org changed reviewers:
+ brettw@chromium.org

[brettw, 2014-10-15 21:11:20]

lgtm

https://codereview.chromium.org/654333002/diff/20001/tools/gn/command_args.cc
File tools/gn/command_args.cc (right):

https://codereview.chromium.org/654333002/diff/20001/tools/gn/command_args.cc...
tools/gn/command_args.cc:199: // Prefer $VISUAL, then $EDITOR, then vi.
Maybe just remove this comment?

[Elliot Glaysher, 2014-10-15 21:21:38]

The CQ bit was checked by erg@chromium.org

[commit-bot: I haz the power, 2014-10-15 21:23:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/654333002/40001

[commit-bot: I haz the power, 2014-10-16 00:56:50]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2014-10-16 00:57:30]

Patchset 3 (id:??) landed as
https://crrev.com/a053ef69be56c9d073a8e383491e0f0854240250
Cr-Commit-Position: refs/heads/master@{#299798}