Make GN use_custom_libcxx work properly with sanitizers.

Make GN use_custom_libcxx work properly with sanitizers.

Its default value depended on a previous flag, but the value of that flag wasn't getting set until the end of the declare_args block. The solution is to add a new declare_args block.

Added documentation for this since it can be surprising.

Committed: https://crrev.com/cc637eee86e6041ac9df3ad7d3160ceb2a88cddd
Cr-Commit-Position: refs/heads/master@{#352109}

[brettw, 2015-10-02 18:31:53]

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

[brettw, 2015-10-02 18:31:55]

The CQ bit was checked by brettw@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-02 18:33:52]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1386713002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1386713002/1

[Bons, 2015-10-02 18:55:22]

lgtm with comment

https://codereview.chromium.org/1386713002/diff/1/tools/gn/functions.cc
File tools/gn/functions.cc (right):

https://codereview.chromium.org/1386713002/diff/1/tools/gn/functions.cc#newco...
tools/gn/functions.cc:352: "   2. At the end of the executing the block, any
variables set within\n"
"at the end of the executing the block"...?

[brettw, 2015-10-02 19:35:57]

The CQ bit was checked by brettw@chromium.org

[brettw, 2015-10-02 19:35:57]

The patchset sent to the CQ was uploaded after l-g-t-m from
andybons@chromium.org
Link to the patchset: https://codereview.chromium.org/1386713002/#ps20001
(title: "Typo")

[commit-bot: I haz the power, 2015-10-02 19:37:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1386713002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1386713002/20001

[commit-bot: I haz the power, 2015-10-02 20:09:49]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-02 20:10:25]

Patchset 2 (id:??) landed as
https://crrev.com/cc637eee86e6041ac9df3ad7d3160ceb2a88cddd
Cr-Commit-Position: refs/heads/master@{#352109}