Add Paths as first-class types in configs.

scripts/slave/recipes/android_webview_aosp.py

Index: scripts/slave/recipes/android_webview_aosp.py
diff --git a/scripts/slave/recipes/android_webview_aosp.py b/scripts/slave/recipes/android_webview_aosp.py
index 29d8290320ca0001df10a827ee58139b7a5ac860..1b45671788dcb07c580e8e81191139aa8e4e1001 100644
--- a/scripts/slave/recipes/android_webview_aosp.py
+++ b/scripts/slave/recipes/android_webview_aosp.py
@@ -37,25 +37,27 @@ def GenTests(api):
   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()