Execute the script using python api module.

step api module does not append python while executing a script, because
of this the steps were throwing exeception on windows platform.
Now we use python api module to execute scripts
BUG=491470

Review URL: https://codereview.chromium.org/1149013006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/build@295398 0039d316-1c4b-4281-b951-d872f2087c98
11 files changed