[iOS] Background colour for UITextView on popover

This change sets clear background colour for UITextView on popover.

Bug: 1108741
Change-Id: Ifa46a2520e6b176fcd3873eeb56c33150a70178e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316196Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Commit-Queue: Viktor Semeniuk <vsemeniuk@google.com>
Cr-Commit-Position: refs/heads/master@{#791241}

ios/chrome/common/ui/elements/popover_label_view_controller.mm

@@ -99,6 +99,7 @@ constexpr CGFloat kVerticalDistance = 24;
   textView.scrollEnabled = NO;
   textView.editable = NO;
   textView.delegate = self;
+  textView.backgroundColor = [UIColor clearColor];
   textView.font = [UIFont preferredFontForTextStyle:UIFontTextStyleSubheadline];
   textView.adjustsFontForContentSizeCategory = YES;
   textView.translatesAutoresizingMaskIntoConstraints = NO;