Convert FieldTrialActivated to use mojo.

chrome/common/field_trial_recorder.mojom

Index: chrome/common/field_trial_recorder.mojom
diff --git a/chrome/common/field_trial_recorder.mojom b/chrome/common/field_trial_recorder.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..007439d7a17735d999af9f9a0bf2ffc98b441263
--- /dev/null
+++ b/chrome/common/field_trial_recorder.mojom
@@ -0,0 +1,11 @@
+// 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.
+
+module chrome.mojom;
+
+// Records field trial activations from the renderers.
+interface FieldTrialRecorder {
+  // Indicates that a field trial has been activated.
+  FieldTrialActivated(string trial_name);
+};