The solution for this is to make all Google Wallet Instant Buy calls from secure pages.
So in your example, the page where "Buy With Google" button is rendered (where you have the snippet) should also be a https page.
EDIT: And the documentation is also updated. refer to https://developers.google.com/commerce/wallet/instant-buy/web/tutorial#call_authorize_to_determine_checkout_page_behavior (see Important note).