Serve mojo WebUI resources from the same origin as the WebUI itself.

content/renderer/web_ui_mojo_context_state.h

Index: content/renderer/web_ui_mojo_context_state.h
diff --git a/content/renderer/web_ui_mojo_context_state.h b/content/renderer/web_ui_mojo_context_state.h
index 4aa12e5153231c4f9ead9170972564b201be5fa0..28872d4486864aaa77398da71624abff95a20079 100644
--- a/content/renderer/web_ui_mojo_context_state.h
+++ b/content/renderer/web_ui_mojo_context_state.h
@@ -62,6 +62,9 @@ class WebUIMojoContextState : public gin::ModuleRegistryObserver {
       const std::string& id,
       const std::vector<std::string>& dependencies) override;
 
+  // Returns the prefix to use for all module requests.
+  const std::string& GetModulePrefix();
+
   // Frame script is executed in. Also used to download resources.
   blink::WebFrame* frame_;
 
@@ -77,6 +80,9 @@ class WebUIMojoContextState : public gin::ModuleRegistryObserver {
   // Set of modules we've fetched script from.
   std::set<std::string> fetched_modules_;
 
+  // A cached copy of the prefix to use for all module requests.
+  std::string module_prefix_;
+
   DISALLOW_COPY_AND_ASSIGN(WebUIMojoContextState);
 };