Convert ChromeViewMsg_SetFieldTrialGroup to use mojo.

chrome/common/renderer_configuration.mojom

Index: chrome/common/renderer_configuration.mojom
diff --git a/chrome/common/renderer_configuration.mojom b/chrome/common/renderer_configuration.mojom
index 851167c743aba7cd62f0bb32fc96680069d4674b..b7679325b3c05069e3ae7b77f8fc2f27981d7e55 100644
--- a/chrome/common/renderer_configuration.mojom
+++ b/chrome/common/renderer_configuration.mojom
@@ -15,4 +15,15 @@ interface RendererConfiguration {
   // 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);
 };