[ios] Handle gracefully WK messages sent by iframes.

ios/web/web_state/ui/crw_web_controller.mm

Index: ios/web/web_state/ui/crw_web_controller.mm
diff --git a/ios/web/web_state/ui/crw_web_controller.mm b/ios/web/web_state/ui/crw_web_controller.mm
index ab0edf30873f5ba62ead08f8d0034afc15094cac..894229110ca7994ba8781d3363be5289655ce03c 100644
--- a/ios/web/web_state/ui/crw_web_controller.mm
+++ b/ios/web/web_state/ui/crw_web_controller.mm
@@ -2685,7 +2685,11 @@ const NSTimeInterval kSnapshotOverlayTransition = 0.5;
 }
 
 - (BOOL)respondToWKScriptMessage:(WKScriptMessage*)scriptMessage {
-  CHECK(scriptMessage.frameInfo.mainFrame);
+  if (!scriptMessage.frameInfo.mainFrame) {
+    // Messages from iframes are not supported yet.

[Jackie Quinn, 2016/11/23 19:17:40]

super nit: s/not supported yet/not currently supported. Reduces the expectation
that it will be supported in some particular timeframe. If there is a timeline
for support, maybe add a TODO and bug.

[Eugene But (OOO till 7-30), 2016/11/23 19:28:39]

Updated comment. The future of iframe injections is unclear, so I'd prefer to
avoid TODO :).

+    return NO;
+  }
+
   int errorCode = 0;
   std::string errorMessage;
   std::unique_ptr<base::Value> inputJSONData(