title: Why put Cart in MaskedWalletRequest?
body: <p>In the docs, when building the <code>MaskedWalletRequest</code> they say:</p>
  
  <blockquote>
    <p>At this point, you won't have the user's chosen shipping address, so
    you'll need to create an estimate of the shipping costs and tax. If
    you set the shopping cart as shown below (highly recommended), make
    sure the cart total matches the sum of the line items added to the
    cart.</p>
  </blockquote>
  
  <p><a href="https://developers.google.com/android-pay/android/tutorial#create_a_masked_wallet_request" rel="nofollow">link</a></p>
  
  <p>Later when I create my <code>FullWalletRequest</code> I have to pass in a <code>Cart</code>*. So why would I want to add a <code>Cart</code> to my <code>MaskedWalletRequest</code>? Why is it "highly recommended"? </p>
  
  <p>*If I don't pass in a <code>Cart</code> I'll get <code>WalletConstants.ERROR_CODE_INVALID_PARAMETERS</code></p>