Adds Mojo IDL.

third_party/WebKit/Source/core/mojo/BUILD.gn

Index: third_party/WebKit/Source/core/mojo/BUILD.gn
diff --git a/third_party/WebKit/Source/core/mojo/BUILD.gn b/third_party/WebKit/Source/core/mojo/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..d28d6fed806523c7e4737d98b8ea45e2bf17414f
--- /dev/null
+++ b/third_party/WebKit/Source/core/mojo/BUILD.gn
@@ -0,0 +1,20 @@
+# 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.
+
+import("//third_party/WebKit/Source/core/core.gni")
+
+blink_core_sources("mojo") {
+  sources = [
+    "Mojo.cpp",
+    "Mojo.h",
+    "MojoHandle.cpp",
+    "MojoHandle.h",
+    "MojoWatcher.cpp",
+    "MojoWatcher.h",
+  ]
+
+  deps = [
+    "//mojo/public/cpp/system",
+  ]
+}