[iOS][Alert] Remove background view

As mentioned by kkhorimoto@ the background view is not needed. And the
chrome shouldn't be constraint to the safe areas, but cover the
presenting view controller.

Bug: 951300
Change-Id: I3097c8dc6ceaaf60c1d0f6ad817a87ef910e5991
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574799Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652528}

ios/chrome/browser/ui/alert_view_controller/alert_view_controller.mm

@@ -88,12 +88,8 @@ constexpr CGFloat kMinimumMargin = 4;
 
 - (void)loadView {
   [super loadView];
-  UIView* grayBackground = [[UIView alloc] init];
-  grayBackground.backgroundColor =
+  self.view.backgroundColor =
       [[UIColor blackColor] colorWithAlphaComponent:kBackgroundAlpha];
-  grayBackground.translatesAutoresizingMaskIntoConstraints = NO;
-  [self.view addSubview:grayBackground];
-  AddSameConstraints(grayBackground, self.view.safeAreaLayoutGuide);
 
   self.contentView = [[UIView alloc] init];
   self.contentView.backgroundColor = [UIColor whiteColor];