Use consistent background color for WKWebViews.

ios/web/web_state/ui/crw_web_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/web/web_state/ui/crw_web_controller.h"
 
 #import <objc/runtime.h>
 #include <stddef.h>
 
 #include <cmath>
@@ skipping 1114 matching lines @@
 
 - (void)webViewDidChange {
   CHECK(_webUsageEnabled) << "Tried to create a web view while suspended!";
 
   UIView* webView = self.webView;
   DCHECK(webView);
 
   [webView setTag:kWebViewTag];
   [webView setAutoresizingMask:UIViewAutoresizingFlexibleWidth |
                                UIViewAutoresizingFlexibleHeight];
-  [webView setBackgroundColor:[UIColor colorWithWhite:0.2 alpha:1.0]];
 
   // Create a dependency between the |webView| pan gesture and BVC side swipe
   // gestures. Note: This needs to be added before the longPress recognizers
   // below, or the longPress appears to deadlock the remaining recognizers,
   // thereby breaking scroll.
   NSSet* recognizers = [_swipeRecognizerProvider swipeRecognizers];
   for (UISwipeGestureRecognizer* swipeRecognizer in recognizers) {
     [self.webScrollView.panGestureRecognizer
         requireGestureRecognizerToFail:swipeRecognizer];
   }
@@ skipping 2769 matching lines @@
   if ([MIMEType isEqualToString:@"text/html"] ||
       [MIMEType isEqualToString:@"application/xhtml+xml"] ||
       [MIMEType isEqualToString:@"application/xml"]) {
     return web::WEB_VIEW_DOCUMENT_TYPE_HTML;
   }
 
   return web::WEB_VIEW_DOCUMENT_TYPE_GENERIC;
 }
 
 @end