Convert ChromeViewMsg_SetFieldTrialGroup to use mojo.

chrome/common/renderer_configuration.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;
 
 import "components/content_settings/core/common/content_settings.mojom";
 
 // Configures the renderer.
 interface RendererConfiguration {
   // Configures the renderer, queued to send when the render process will
   // launch.
   SetInitialConfiguration(bool is_incognito_process);
 
   // Set the content setting rules stored by the renderer.
   SetContentSettingRules(
       content_settings.mojom.RendererContentSettingRules rules);
+
+  // Tells the renderer to create a FieldTrial, and by using a 100% probability
+  // for the FieldTrial, forces the FieldTrial to have assigned group name.
+  //
+  // Example trial_name / group_name pairs are:
+  //    - "BrowserScheduler" / "Default"
+  //    - "QUIC" / "Enabled"
+  //    - "SimpleCacheTrial" / "ExperimentYes"
+  //    - "WebFontsInterventionV2" / "Enabled-slow2g"
+  //    - "MarkNonSecureAs" / "show-non-secure-passwords-cc-ui"

[Alexei Svitkine (slow), 2017/02/02 17:29:50]

I'm not sure the examples are very useful here. If you do want to keep them, I
suggest just limiting it to the first couple.

[Tom Sepez, 2017/02/02 18:42:51]

Nit: Are there any rules about what's allowed (e.g punctuation, charset, hi-byte
chars, etc)?  Otherwise I wouldn't elaborate at all.

[nigeltao1, 2017/02/03 01:52:49]

I've replaced this with:
// See base/metrics/field_trial.h for more information.

+  SetFieldTrialGroup(string trial_name, string group_name);
 };