[ios] Moves find-in-page code out of Tab and into FindTabHelper.

ios/chrome/browser/find_in_page/find_in_page_controller.mm

 // Copyright 2012 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.
 
 #import "ios/chrome/browser/find_in_page/find_in_page_controller.h"
 
 #import <UIKit/UIKit.h>
 
 #import <cmath>
 #include <memory>
@@ skipping 21 matching lines @@
 namespace {
 // The delay (in secs) after which the find in page string will be pumped again.
 const NSTimeInterval kRecurringPumpDelay = .01;
 
 // Keeps find in page search term to be shared between different tabs. Never
 // reset, not stored on disk.
 static NSString* gSearchTerm;
 }
 
 @interface FindInPageController () <DOMAltering, CRWWebStateObserver>
-// The find in page controller delegate.
+// The find in page controller delegate.  Can be nil.
 @property(nonatomic, readonly) id<FindInPageControllerDelegate> delegate;
 
 // The web view's scroll view.
 - (CRWWebViewScrollViewProxy*)webViewScrollView;
 // Find in Page text field listeners.
 - (void)findBarTextFieldWillBecomeFirstResponder:(NSNotification*)note;
 - (void)findBarTextFieldDidResignFirstResponder:(NSNotification*)note;
 // Keyboard listeners.
 - (void)keyboardDidShow:(NSNotification*)note;
 - (void)keyboardWillHide:(NSNotification*)note;
@@ skipping 43 matching lines @@
 }
 
 + (NSString*)searchTerm {
   return gSearchTerm;
 }
 
 - (id)initWithWebState:(web::WebState*)webState
               delegate:(id<FindInPageControllerDelegate>)delegate {
   self = [super init];
   if (self) {
-    DCHECK(delegate);
     _findInPageJsManager = base::mac::ObjCCastStrict<JsFindinpageManager>(
         [webState->GetJSInjectionReceiver()
             instanceOfClass:[JsFindinpageManager class]]);
     _delegate = delegate;
     _webStateObserverBridge.reset(
         new web::WebStateObserverBridge(webState, self));
     _webViewProxy = webState->GetWebViewProxy();
     [[NSNotificationCenter defaultCenter]
         addObserver:self
            selector:@selector(findBarTextFieldWillBecomeFirstResponder:)
@@ skipping 258 matching lines @@
 
 - (BOOL)canReleaseDOMLock {
   return NO;
 }
 
 - (void)releaseDOMLockWithCompletionHandler:(ProceduralBlock)completionHandler {
   NOTREACHED();
 }
 
 @end