Make sure abs-path gn labels have separate target_gen_dirs

Make sure abs-path gn labels have separate target_gen_dirs

Use gen/ABS_PATH/... for target_gen_dir of build files outside of the
source tree, to mimic obj/ABS_PATH that's used for object directories
for such build files.

Previously, all build files outside of the source tree would use
root_gen_dir (e.g. out/Default/gen) as target_gen_dir. This broke the
"target_gen_dir is unique between different BUILD.gn files" assumption,
causing strange behavior and "ninja: multiple targets generate ..."
warnings for users of absolute path BUILD.gn files outside of //.

BUG=445454
R=brettw@chromium.org

Committed: https://crrev.com/5b6813c06deae18ca309fba9913ee18e28150122
Cr-Commit-Position: refs/heads/master@{#356011}

[tsniatowski, 2015-10-23 20:06:31]

PTAL

[brettw, 2015-10-26 00:36:52]

lgtm

https://codereview.chromium.org/1420973003/diff/1/tools/gn/filesystem_utils.cc
File tools/gn/filesystem_utils.cc (right):

https://codereview.chromium.org/1420973003/diff/1/tools/gn/filesystem_utils.c...
tools/gn/filesystem_utils.cc:689: void AppendFixedAbsolutePathSuffix(const
BuildSettings *build_settings,
Nit: * next to type instead like your other ones.

[tsniatowski, 2015-10-26 06:21:43]

The CQ bit was checked by tsniatowski@opera.com

[tsniatowski, 2015-10-26 06:21:43]

The patchset sent to the CQ was uploaded after l-g-t-m from brettw@chromium.org
Link to the patchset: https://codereview.chromium.org/1420973003/#ps20001
(title: " ")

[commit-bot: I haz the power, 2015-10-26 06:21:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1420973003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1420973003/20001

[commit-bot: I haz the power, 2015-10-26 06:32:26]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-26 06:33:02]

Patchset 2 (id:??) landed as
https://crrev.com/5b6813c06deae18ca309fba9913ee18e28150122
Cr-Commit-Position: refs/heads/master@{#356011}