Set WKScrollView.contentInsetAdjustmentBehavior to |never|

This is required for //ios/web_view (crbug.com/774876).

TEST=Fullscreen and overscroll actions

Bug: 774876
Change-Id: Ie6c57ea5efd2d7fc6d8f26117db07171c085ad69
Reviewed-on: https://chromium-review.googlesource.com/720856
Commit-Queue: Hiroshi Ichikawa <ichikawa@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509259}

ios/web/web_state/web_view_internal_creation_util.mm

@@ -57,6 +57,11 @@ WKWebView* BuildWKWebView(CGRect frame,
   // reasonable value.
   web_view.scrollView.decelerationRate = UIScrollViewDecelerationRateNormal;
 
+  if (@available(iOS 11.0, *)) {
+    web_view.scrollView.contentInsetAdjustmentBehavior =
+        UIScrollViewContentInsetAdjustmentNever;
+  }
+
   if (context_menu_delegate) {
     CRWContextMenuController* context_menu_controller = [
         [CRWContextMenuController alloc] initWithWebView:web_view