Normalize output path to cp.py

Normalize output path to cp.py

The js unit test targets have subtle dependencies on sources.
Specifically, a rule pulls in js files and generates/runs js tests for
them. This can be an issue with a relative path when the commands are
run in parallel because a relative path may not exist. In this case, the
rule for cr.js which is in ui depends on chrome. An easy way to repro
this failure is:

$ ninja -C out/Debug unit_tests
$ rm -rf out/Debug/test_data
$ ninja -C out/Debug test_data/chrome/../ui/webui/resources/js/cr.js

Ninja is fine because it normalizes the path early enough whereas in
python we relied on the system to compute the path but out/Debug/test_data/chrome may not exist

BUG=235731
Hat-tip to cjhopman for help debugging.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=197224

[cjhopman, 2013-04-29 22:52:32]

lgtm

[commit-bot: I haz the power, 2013-04-29 22:55:32]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/yfriedman@chromium.org/14432003/1

[commit-bot: I haz the power, 2013-04-30 01:57:12]

Change committed as 197224