Check if the JS_ENGINE key exists as an environment variable before reading it.

  # JS_ENGINE environment  variable, which is used by the build system in

  # external/webkit/Android.mk.

  js_engine = options.js_engine

  if not js_engine:

  if not js_engine and os.environ.has_key('JS_ENGINE'):

    js_engine = os.environ['JS_ENGINE']

  if js_engine:

    run_layout_test_cmd_postfix += " -e jsengine " + js_engine