tools/gn: mark first TestTarget parameter as const

tools/gn: mark first TestTarget parameter as const

Style guide mandates that parameters passed by reference must be labeled
'const'. That, or they should be const T*.

https://google.github.io/styleguide/cppguide.html#Reference_Arguments

This is also to please cpplint complain on it:
./tools/gn/test_with_scope.h:104:  Is this a non-const reference? If so,
make const or use a pointer: TestWithScope& setup [runtime/references] [2]

BUG=None
TEST=gn_unittests
R=scottmg@chromium.org

Committed: https://crrev.com/45e4d2956883c02a20e0269ae74b24923a34cbc5
Cr-Commit-Position: refs/heads/master@{#360251}

[tfarina, 2015-11-17 21:31:20]

PTAL.

[scottmg, 2015-11-17 21:39:31]

lgtm

[tfarina, 2015-11-18 01:04:24]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-11-18 01:08:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1453843003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1453843003/1

[commit-bot: I haz the power, 2015-11-18 01:23:41]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-18 01:24:38]

Patchset 1 (id:??) landed as
https://crrev.com/45e4d2956883c02a20e0269ae74b24923a34cbc5
Cr-Commit-Position: refs/heads/master@{#360251}