[ios] Ensure MDCAppBarViewController's view matches the width of...

[ios] Ensure MDCAppBarViewController's view matches the width of PaymentRequestPickerViewController's view.

Late addition to the docs: https://github.com/material-components/material-components-ios/tree/develop/components/AppBar#migration-guide-mdcappbar-to-mdcappbarviewcontroller

Bug: 874471
Change-Id: If17ea9de20fec0463e918aea4f401427c3572bc5
Reviewed-on: https://chromium-review.googlesource.com/1239008Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594088}

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

@@ -138,6 +138,12 @@ NSString* const kPaymentRequestPickerSearchBarAccessibilityID =
   [self addChildViewController:self.appBarViewController];
   ConfigureAppBarViewControllerWithCardStyle(self.appBarViewController);
   self.appBarViewController.headerView.trackingScrollView = self.tableView;
+  // Match the width of the parent view.
+  CGRect frame = self.appBarViewController.view.frame;
+  frame.origin.x = 0;
+  frame.size.width =
+      self.appBarViewController.parentViewController.view.bounds.size.width;
+  self.appBarViewController.view.frame = frame;
   [self.view addSubview:self.appBarViewController.view];
   [self.appBarViewController didMoveToParentViewController:self];
 }