I think we should try to fix this sooner rather than later, as it's definitely a source of weird bugs and user questions.

Can I work on this or is it already handled by someone else?

@tim@rnc-ag.de - I don't think anyone else is actively working on this, so feel free to give it a shot.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5662a8082448584fd3d77f0dba03759b2163e8e5

commit 5662a8082448584fd3d77f0dba03759b2163e8e5
Author: tim <tim@rnc-ag.de>
Date: Tue Jul 12 00:07:46 2016

GN: Don't define argument overrides globally

Previously, argument overrides (set via "gn args" or toolchain_args()) were
global, even though their default values (set inside declare_args()) were set
locally and thus only available to the file that set them, plus any file that
imported it.

Now, argument overrides are always applied locally, at the end of the
declare_args() block. This ensures that argument defaults and overrides
behave in the same way.

BUG= 619963 

Review-Url: https://codereview.chromium.org/2092623002
Cr-Commit-Position: refs/heads/master@{#404746}

[modify] https://crrev.com/5662a8082448584fd3d77f0dba03759b2163e8e5/tools/gn/args.cc
[modify] https://crrev.com/5662a8082448584fd3d77f0dba03759b2163e8e5/tools/gn/args.h
[modify] https://crrev.com/5662a8082448584fd3d77f0dba03759b2163e8e5/tools/gn/args_unittest.cc