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..da960e2040e3356c04359783bd5825b7fbc6ca32 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 +42,13 @@ 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.
+    if (channel) {
+      chrome::mojom::FieldTrialGroupSetterAssociatedPtr ftgs_interface;

[nigeltao1, 2017/01/20 00:35:59]

I dropped some temporary printf's here, and it looks like there's 20 to 30 of
these calls during start-up, and another 2 or 3 trickle in as I browsed a few
sites. I don't think we're at the scale yet that we need to re-use a single
binding instead of continually bringing up and tearing down multiple ones, or
somehow batching the field trial group setter calls. 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.

[Sam McNally, 2017/01/20 02:35:38]

20 to 30 notifications or 20 to 30 messages? Ideally, we would avoid slowing
down startup.

Also, no abbreviations.

+      channel->GetRemoteAssociatedInterface(&ftgs_interface);
+      ftgs_interface->SetFieldTrialGroup(field_trial_name, group_name);
+    }
   }
 }