Fix white expiration picker with dark mode by using clear color.

I'm not entirely sure why this fixes the issue, but after a couple hours
it was all I was able to come up with. Interestingly, if I move the line
inside the picker's init, the issue remains.

https://screenshot.googleplex.com/UzOWW6uOVRE.png
https://screenshot.googleplex.com/XPG9EPkODoP.png

Bug: 1028090
Change-Id: I8b6ce7135356c863aa396b238ce9a51b0dac26f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1940955Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Alfonso Garza <alfonsogarza@google.com>
Auto-Submit: Alfonso Garza <alfonsogarza@google.com>
Cr-Commit-Position: refs/heads/master@{#720602}

ios/chrome/browser/ui/autofill/card_expiration_date_fix_flow_view_bridge.mm

@@ -149,6 +149,7 @@ void CardExpirationDateFixFlowViewBridge::DeleteSelf() {
 
   _expirationDatePicker =
       [[ExpirationDatePicker alloc] initWithFrame:CGRectZero];
+  _expirationDatePicker.backgroundColor = [UIColor clearColor];
   __weak CardExpirationDateFixFlowViewController* weakSelf = self;
   _expirationDatePicker.onDateSelected = ^(NSString* month, NSString* year) {
     CardExpirationDateFixFlowViewController* strongSelf = weakSelf;