Remove reference to nonexistent "register_callback_overwrites_"

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I5eddc8605d1088b275edba9344c0184e67de6f2f
Reviewed-on: https://chromium-review.googlesource.com/757064
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518191}

content/public/browser/web_ui.h

@@ -70,8 +70,7 @@ class CONTENT_EXPORT WebUI {
       std::unique_ptr<WebUIMessageHandler> handler) = 0;
 
   // Used by WebUIMessageHandlers. If the given message is already registered,
-  // the call has no effect unless |register_callback_overwrites_| is set to
-  // true.
+  // the call has no effect.
   typedef base::Callback<void(const base::ListValue*)> MessageCallback;
   virtual void RegisterMessageCallback(const std::string& message,
                                        const MessageCallback& callback) = 0;

ios/web/public/webui/web_ui_ios.h

@@ -47,8 +47,7 @@ class WebUIIOS {
       std::unique_ptr<WebUIIOSMessageHandler> handler) = 0;
 
   // Used by WebUIIOSMessageHandlers. If the given message is already
-  // registered, the call has no effect unless |register_callback_overwrites_|
-  // is set to true.
+  // registered, the call has no effect.
   typedef base::Callback<void(const base::ListValue*)> MessageCallback;
   virtual void RegisterMessageCallback(const std::string& message,
                                        const MessageCallback& callback) = 0;