Convert ChromeViewMsg_SetFieldTrialGroup to use mojo.

chrome/common/field_trial.mojom

Index: chrome/common/field_trial.mojom
diff --git a/chrome/common/field_trial_recorder.mojom b/chrome/common/field_trial.mojom
similarity index 56%
rename from chrome/common/field_trial_recorder.mojom
rename to chrome/common/field_trial.mojom
index 007439d7a17735d999af9f9a0bf2ffc98b441263..6eea4bb15f3a5257c19b351acdc2cdd6d2852e42 100644
--- a/chrome/common/field_trial_recorder.mojom
+++ b/chrome/common/field_trial.mojom
@@ -4,6 +4,13 @@
 
 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.