Convert ChromeViewMsg_SetFieldTrialGroup to use mojo.

chrome/browser/metrics/field_trial_synchronizer.cc

Index: chrome/browser/metrics/field_trial_synchronizer.cc
diff --git a/chrome/browser/metrics/field_trial_synchronizer.cc b/chrome/browser/metrics/field_trial_synchronizer.cc
index 842f25db6d6156577dc889f860a1964e362a74b8..450b663099e8525792252439d227de53b59ef7a6 100644
--- a/chrome/browser/metrics/field_trial_synchronizer.cc
+++ b/chrome/browser/metrics/field_trial_synchronizer.cc
@@ -7,7 +7,7 @@
 #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"
@@ -42,8 +42,15 @@ void FieldTrialSynchronizer::NotifyAllRenderers(
   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);
+    }
   }
 }