[iOS][MF] Tweaks to the Manual Fallback popover

Sets the background color to white.
Changes the preffered height to 360.

Bug: 878388, 845472
Change-Id: If4ff0363808af7c373e4f70d9d664196867fdbb6
Reviewed-on: https://chromium-review.googlesource.com/c/1350967Reviewed-by: Yi Su <mrsuyi@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611632}

ios/chrome/browser/ui/autofill/manual_fill/fallback_coordinator.mm

@@ -70,6 +70,7 @@ initWithBaseViewController:(UIViewController*)viewController
   popoverPresentationController.permittedArrowDirections =
       UIPopoverArrowDirectionUp | UIMenuControllerArrowDown;
   popoverPresentationController.delegate = self;
+  popoverPresentationController.backgroundColor = UIColor.whiteColor;
 }
 
 #pragma mark - ChromeCoordinator

ios/chrome/browser/ui/autofill/manual_fill/fallback_view_controller.mm

@@ -27,7 +27,7 @@ namespace {
 constexpr float PopoverPreferredWidth = 320;
 
 // This is the maximum height used for |self.preferredContentSize|.
-constexpr float PopoverMaxHeight = 250;
+constexpr float PopoverMaxHeight = 360;
 
 }  // namespace