[ios] Removed keyCode from FormActivityRegistered API.

ios/web/web_state/web_state_observer.mm

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/web/public/web_state/web_state_observer.h"
 
 #include "ios/web/public/load_committed_details.h"
 #include "ios/web/public/web_state/web_state.h"
 
 namespace web {
 
-int WebStateObserver::kInvalidFormKeyCode = -1;
-
 WebStateObserver::WebStateObserver(WebState* web_state) : web_state_(nullptr) {
   Observe(web_state);
 }
 
 WebStateObserver::WebStateObserver() : web_state_(nullptr) {}
 
 WebStateObserver::~WebStateObserver() {
   if (web_state_)
     web_state_->RemoveObserver(this);
 }
 
 void WebStateObserver::Observe(WebState* web_state) {
   if (web_state == web_state_) {
     // Early exit to avoid infinite loops if we're in the middle of a callback.
     return;
   }
   if (web_state_)
     web_state_->RemoveObserver(this);
   web_state_ = web_state;
   if (web_state_)
     web_state_->AddObserver(this);
 }
 
 void WebStateObserver::ResetWebState() {
   web_state_->RemoveObserver(this);
   web_state_ = nullptr;
 }
 
 }  // namespace web