Wrap ninja invocation to include python in path, and set up compiler environment before invoking the ninja binary itself (which is moved to a subdirectory).
Review URL: https://chromiumcodereview.appspot.com/9967020

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@132239 0039d316-1c4b-4281-b951-d872f2087c98