Add a wrapper script for GN to depot_tools

Add a wrapper script for GN to depot_tools

This automatically searches the path for a source root (or takes one from the
command line), and then executes the current platform's GN binary inside that
source tree.

This will allow the user to have a "gn" command on their path, and allow our
scripts to run GN without having to bake-in the logic for finding the correct
platform's GN binary.

[M-A Ruel, 2013-11-08 00:48:31]

a few comments but lgtm

https://codereview.chromium.org/66013002/diff/30001/gn.py
File gn.py (right):

https://codereview.chromium.org/66013002/diff/30001/gn.py#newcode6
gn.py:6: # This script is a wrapper around the GN binary that is pulled from
Google
Make that a docstring

"""Wrapper around the GN binary that ...

https://codereview.chromium.org/66013002/diff/30001/gn.py#newcode10
gn.py:10: # This script makes there be one place that we can forward to the
correct
unusual English structure to me. I should read more.

https://codereview.chromium.org/66013002/diff/30001/gn.py#newcode18
gn.py:18: 
2 lines between file level symbols

https://codereview.chromium.org/66013002/diff/30001/gn.py#newcode31
gn.py:31: tree and returns the found path. Returns nil if no source root could
s/nil/None/

https://codereview.chromium.org/66013002/diff/30001/gn.py#newcode52
gn.py:52: raise PlatformUnknownError('Unknown platform for GN: ' + sys.platform)
I'd recommend to just print >> sys.stderr and return 1.

Using exceptions is useful if you plan to use this file as a library. Otherwise
it just prints an unneeded stack trace to the user.

https://codereview.chromium.org/66013002/diff/30001/gn.py#newcode64
gn.py:64: sourceroot = None
I think it is not a bad idea to not support this at all. Like with git, the idea
is that you make sure the cwd is the proper directory you want to use, that's
it.

I feel this would make the code less brittle long-term and set expectation
clearly, e.g. "you need to cd to the source tree first" but I'm not strongly
opposed to this.

https://codereview.chromium.org/66013002/diff/30001/gn.py#newcode73
gn.py:73: raise SourceTreeNotFoundError(
I'd recommend to just print >> sys.stderr and return 1.

[brettw, 2013-11-08 20:49:16]

https://codereview.chromium.org/66013002/diff/30001/gn.py
File gn.py (right):

https://codereview.chromium.org/66013002/diff/30001/gn.py#newcode52
gn.py:52: raise PlatformUnknownError('Unknown platform for GN: ' + sys.platform)
I kept this exception. I'm going to use this as a library for gyp_chromium, I
think. I replaced the other exception since that's only for the command-line
facing interface.

https://codereview.chromium.org/66013002/diff/30001/gn.py#newcode64
gn.py:64: sourceroot = None
Okay, I removed this.

[commit-bot: I haz the power, 2013-11-08 20:49:23]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/brettw@chromium.org/66013002/80001

[commit-bot: I haz the power, 2013-11-08 20:51:42]

Change committed as 233983