[Payment Request] State field: allow selecting 1st state.

If the state field is unpopulated, inserts a blank element at the
beginning of the list of states, as the first element is selected by
default when the UIPickerView is shown.

R=mahmadi

Bug: 767972
Change-Id: Iaa0c4bc29b863d8c521f3e9f0b826eb7ccef408d
Reviewed-on: https://chromium-review.googlesource.com/689857Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Marc-Antoine Courteau <macourteau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505070}

ios/chrome/browser/ui/payments/address_edit_mediator.mm

@@ -175,7 +175,15 @@
   // Notify the view controller asynchronously to allow for the view to update.
   __weak AddressEditMediator* weakSelf = self;
   dispatch_async(dispatch_get_main_queue(), ^{
-    [weakSelf.consumer setOptions:@[ [regions allKeys] ]
+    NSMutableArray<NSString*>* values =
+        [[NSMutableArray alloc] initWithArray:[regions allKeys]];
+    // If the field contains no value, insert an empty value at the beginning
+    // of the list of options, as the first option is selected when the UI
+    // opens. Otherwise, it would be impossible for the user to select the first
+    // option without selecting another option first.
+    if (!weakSelf.regionField.value)
+      [values insertObject:@"" atIndex:0];
+    [weakSelf.consumer setOptions:@[ values ]
                    forEditorField:weakSelf.regionField];
   });
 }