Use explicit WebString conversions in remaining content files

content/renderer/render_frame_proxy.cc

Index: content/renderer/render_frame_proxy.cc
diff --git a/content/renderer/render_frame_proxy.cc b/content/renderer/render_frame_proxy.cc
index 843f030d952016852709f69a44b0fccf1cb35645..c080cefcba6138b7a4399ef654a83a6841bb173d 100644
--- a/content/renderer/render_frame_proxy.cc
+++ b/content/renderer/render_frame_proxy.cc
@@ -459,10 +459,10 @@ void RenderFrameProxy::forwardPostMessage(
 
   FrameMsg_PostMessage_Params params;
   params.is_data_raw_string = false;
-  params.data = event.data().toString();
-  params.source_origin = event.origin();
+  params.data = event.data().toString().utf16();
+  params.source_origin = event.origin().utf16();
   if (!target_origin.isNull())
-    params.target_origin = target_origin.toString();
+    params.target_origin = target_origin.toString().utf16();
 
   params.message_ports =
       WebMessagePortChannelImpl::ExtractMessagePortIDs(event.releaseChannels());