Adds Mojo IDL.

third_party/WebKit/Source/core/mojo/MojoWatcher.idl

Index: third_party/WebKit/Source/core/mojo/MojoWatcher.idl
diff --git a/third_party/WebKit/Source/core/mojo/MojoWatcher.idl b/third_party/WebKit/Source/core/mojo/MojoWatcher.idl
new file mode 100644
index 0000000000000000000000000000000000000000..0c66d3fa815fee9004864cde5ff1fd3d5ebc43ef
--- /dev/null
+++ b/third_party/WebKit/Source/core/mojo/MojoWatcher.idl
@@ -0,0 +1,11 @@
+// Copyright 2017 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.
+
+[
+    ActiveScriptWrappable,
+    DependentLifetime,
+    RuntimeEnabled=MojoJS,
+] interface MojoWatcher {
+    MojoResult cancel();
+};