Add leakcanary to DEPS and third_party

third_party/leakcanary/config.gni

Index: third_party/leakcanary/config.gni
diff --git a/testing/legion/lib/__init__.py b/third_party/leakcanary/config.gni
similarity index 77%
copy from testing/legion/lib/__init__.py
copy to third_party/leakcanary/config.gni
index ca3e206fdd8f2b8e5facc81da817abd7d2ca143e..f6adcddf3ecc4442c7793666dfeecaf1f108fea8 100644
--- a/testing/legion/lib/__init__.py
+++ b/third_party/leakcanary/config.gni
@@ -1,3 +1,7 @@
 # Copyright 2016 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.
+
+declare_args() {
+  enable_leakcanary = false
+}