Add Paths as first-class types in configs.

scripts/slave/recipes/android_webview_aosp.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 DEPS = [
   'android',
   'path',
   'properties',
   'rietveld'
 ]
@@ skipping 19 matching lines @@
     build_tool='make-android',
     targets=['libwebviewchromium', 'android_webview_java'],
     use_goma=True)
 
 def GenTests(api):
   yield api.test('basic') + api.properties.scheduled()
 
   yield (
     api.test('uses_android_repo') +
     api.properties.scheduled() +
-    api.path.exists('[SLAVE_BUILD_ROOT]/android-src/.repo/repo/repo')
+    api.path.exists(
+      api.path.slave_build('android-src', '.repo', 'repo', 'repo'))
   )
 
   yield (
     api.test('doesnt_sync_if_android_present') +
     api.properties.scheduled() +
-    api.path.exists('[SLAVE_BUILD_ROOT]/android-src')
+    api.path.exists(api.path.slave_build('android-src'))
   )
 
   yield (
     api.test('does_delete_stale_chromium') +
     api.properties.scheduled() +
-    api.path.exists('[SLAVE_BUILD_ROOT]/android-src/external/chromium_org')
+    api.path.exists(
+      api.path.slave_build('android-src', 'external', 'chromium_org'))
   )
 
   yield (
     api.test('uses_goma_test') +
     api.properties.scheduled() +
-    api.path.exists('[BUILD_ROOT]/goma')
+    api.path.exists(api.path.build('goma'))
   )
 
   yield api.test('works_if_revision_not_present') + api.properties.generic()
 
   yield api.test('trybot') + api.properties.tryserver()