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.

Review URL: https://chromiumcodereview.appspot.com/14432003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@197224 0039d316-1c4b-4281-b951-d872f2087c98