[ios] Remove rootViewController usage from omnibox

The call to |-rootViewController| is used to calculate the size of the
autocomplete popup view in tablet, relative to the keyboard being shown.

We now convert to the mainScreen coordinate space, which is the
intended use of |-rootViewController|.

Bug: 791787
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ib6c8a6de473005de9ec2ce899990653662c1b64f
Reviewed-on: https://chromium-review.googlesource.com/882311
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532002}

ios/chrome/browser/ui/omnibox/omnibox_popup_view_controller.mm

@@ -359,10 +359,9 @@ UIColor* BackgroundColorIncognito() {
 }
 
 - (void)updateContentInsetForKeyboard {
-  UIView* toView =
-      [UIApplication sharedApplication].keyWindow.rootViewController.view;
   CGRect absoluteRect =
-      [self.tableView convertRect:self.tableView.bounds toView:toView];
+      [self.tableView convertRect:self.tableView.bounds
+                toCoordinateSpace:UIScreen.mainScreen.coordinateSpace];
   CGFloat screenHeight = CurrentScreenHeight();
   CGFloat bottomInset = screenHeight - self.tableView.contentSize.height -
                         _keyboardHeight - absoluteRect.origin.y -