Track step nesting in StreamEngine.

recipes/example/nested.py

 # Copyright 2015 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 
 DEPS = [
   'step',
 ]
 
 
 def RunSteps(api):
   # Nest all steps below this.
   with api.step.nest('complicated thing'):
     with api.step.nest('first part'):
-      api.step('wait a bit', ['sleep', '10'])
+      api.step('wait a bit', ['sleep', '1'])
 
     # Prefix the name without indenting.
     with api.step.context({'name': 'attempt number'}):
       step_result = api.step('one', ['echo', 'herpy'])
       assert step_result.step['name'] == 'complicated thing.attempt number.one'
       api.step('two', ['echo', 'derpy'])
 
-  api.step('simple thing', ['sleep', '10'])
+  api.step('simple thing', ['sleep', '1'])
 
 
 def GenTests(api):
   yield api.test('basic')