Convert ChromeViewMsg_SetFieldTrialGroup to use mojo.

chrome/common/field_trial.mojom

 // 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;
 
+// Sets renderers' field trial groups.
+interface FieldTrialGroupSetter {
+  // Tells the renderer to create a FieldTrial, and by using a 100% probability
+  // for the FieldTrial, forces the FieldTrial to have assigned group name.
+  SetFieldTrialGroup(string trial_name, string group_name);
+};
+
 // Records field trial activations from the renderers.
 interface FieldTrialRecorder {
   // Indicates that a field trial has been activated.
   FieldTrialActivated(string trial_name);
 };