path: Add tracking of volatile paths.

recipe_modules/path/example.py

Index: recipe_modules/path/example.py
diff --git a/recipe_modules/path/example.py b/recipe_modules/path/example.py
index 213b07ec40f14fa3cc5ddc3942ab8492378279ac..e3393d514ebb6afcce51f983ecd3d5e6f4235324 100644
--- a/recipe_modules/path/example.py
+++ b/recipe_modules/path/example.py
@@ -55,6 +55,8 @@ def RunSteps(api):
   api.path.mock_add_paths(home_path)
   assert api.path.exists(home_path)
 
+  api.step('volatile tmp_dir', ['echo', api.path.is_volatile('tmp_dir')])
+
   # Convert strings to Paths
   paths_to_convert =  [
     api.path['start_dir'].join('some', 'thing'),
@@ -108,7 +110,8 @@ def GenTests(api):
            api.properties(**{
               '$recipe_engine/path': {
                 'cache_dir': '/c',
-                'tmp_dir': '/t'
+                'tmp_dir': '/t',
+                'volatile': ['tmp_dir'],
               },
            }) +
            api.path.exists(api.path['tmp_base']))