Use config.json to enable Isolated gRPC proxy

client/tests/isolate_storage_test.py

Index: client/tests/isolate_storage_test.py
diff --git a/client/tests/isolate_storage_test.py b/client/tests/isolate_storage_test.py
index e26e4ff750f3fcad43a2e10906853c7fe3a97fcf..14e3e1e1350205bd35d3dc4d913cb04bc184d145 100755
--- a/client/tests/isolate_storage_test.py
+++ b/client/tests/isolate_storage_test.py
@@ -59,9 +59,9 @@ def raiseError(code):
 
 class IsolateStorageTest(auto_stub.TestCase):
   def get_server(self):
-    os.environ['ISOLATED_GRPC_PROXY'] = 'https://luci.com/client/bob'
     return isolate_storage.IsolateServerGrpc('https://luci.appspot.com',
-                                             'default-gzip')
+                                             'default-gzip',
+                                             'https://luci.com/client/bob')
 
   def testFetchHappySimple(self):
     """Fetch: if we get a few chunks with the right offset, everything works"""