Use consistent background color for WKWebViews.

ios/web/web_state/web_view_internal_creation_util.mm

Index: ios/web/web_state/web_view_internal_creation_util.mm
diff --git a/ios/web/web_state/web_view_internal_creation_util.mm b/ios/web/web_state/web_view_internal_creation_util.mm
index cbf01d6474f40f6926eea40971cc5fdf629c8eb4..2ac42ed59d8e0c0fde32f2daa4784b95bd783205 100644
--- a/ios/web/web_state/web_view_internal_creation_util.mm
+++ b/ios/web/web_state/web_view_internal_creation_util.mm
@@ -41,6 +41,9 @@ web::WeakNSObjectCounter& GetActiveWKWebViewCounter() {
   return active_wk_web_view_counter;
 }
 
+// The brightness of the web views' background colors.
+const CGFloat kWebViewBackgroundColorBrightness = 0.2;
+
 // Decides if web views can be created.
 bool gAllowWebViewCreation = NO;
 
@@ -184,6 +187,8 @@ WKWebView* CreateWKWebView(CGRect frame,
 #endif
   WKWebView* result =
       [[WKWebView alloc] initWithFrame:frame configuration:configuration];
+  result.backgroundColor =
+      [UIColor colorWithWhite:kWebViewBackgroundColorBrightness alpha:1.0];
 #if !defined(NDEBUG)
   gAllowWebViewCreation = previous_allow_web_view_creation_value;
 #endif