Convert ChromeViewMsg_SetFieldTrialGroup to use mojo.

chrome/browser/metrics/field_trial_synchronizer.cc

 // Copyright (c) 2012 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.
 
 #include "chrome/browser/metrics/field_trial_synchronizer.h"
 
 #include "base/bind.h"
 #include "base/logging.h"
 #include "base/threading/thread.h"
-#include "chrome/common/render_messages.h"
+#include "chrome/common/field_trial.mojom.h"
 #include "components/variations/variations_util.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/render_process_host.h"
 
 using content::BrowserThread;
 
 namespace {
 
 // This singleton instance should be constructed during the single threaded
 // portion of main(). It initializes globals to provide support for all future
@@ skipping 14 matching lines @@
 void FieldTrialSynchronizer::NotifyAllRenderers(
     const std::string& field_trial_name,
     const std::string& group_name) {
   // To iterate over RenderProcessHosts, or to send messages to the hosts, we
   // need to be on the UI thread.
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
   for (content::RenderProcessHost::iterator it(
           content::RenderProcessHost::AllHostsIterator());
        !it.IsAtEnd(); it.Advance()) {
-    it.GetCurrentValue()->Send(new ChromeViewMsg_SetFieldTrialGroup(
-        field_trial_name, group_name));
+    IPC::ChannelProxy* channel = it.GetCurrentValue()->GetChannel();
+    // channel might be NULL in tests.

[sky, 2017/01/20 17:12:59]

optional: NULL -> null

+    if (channel) {
+      chrome::mojom::FieldTrialGroupSetterAssociatedPtr
+          field_trial_group_setter;
+      channel->GetRemoteAssociatedInterface(&field_trial_group_setter);

[sky, 2017/01/20 17:12:59]

It seems weird to me to configure something via an interface that is promptly
dropped on the floor. I would expect SetFieldTrialGroup to exist on an interface
that is used for fieldtrials.

[nigeltao1, 2017/01/20 22:40:58]

Yeah, I did say

"But let me know if I should look at keeping the binding around for longer than
the method scope, or changing the FieldTrialSynchronizer::NotifyAllRenderers API
from single to batch. Obviously, either of these is more complicated, and leads
to a bigger commit."

earlier in the code review. I'll submit this as is and investigate further.

+      field_trial_group_setter->SetFieldTrialGroup(field_trial_name,
+                                                   group_name);
+    }
   }
 }
 
 void FieldTrialSynchronizer::OnFieldTrialGroupFinalized(
     const std::string& field_trial_name,
     const std::string& group_name) {
   BrowserThread::PostTask(
       BrowserThread::UI, FROM_HERE,
       base::Bind(&FieldTrialSynchronizer::NotifyAllRenderers,
                  this,
                  field_trial_name,
                  group_name));
   variations::SetVariationListCrashKeys();
 }
 
 FieldTrialSynchronizer::~FieldTrialSynchronizer() {
   base::FieldTrialList::RemoveObserver(this);
   g_field_trial_synchronizer = NULL;
 }