It's not often discussed in these terms, but validity is a relative concept. A document is valid or invalid only with respect to the requirements of a particular specification.
In this case, the microdata spec says nothing about WebPage, so the inclusion of the itemprop in an implicit WebPage item has no effect on the validity of your document with respect to the Microdata spec.
On the other hand, with respect to the schema.org WebPage spec, your document, or at least the WebPage item within it, would be invalid.
Whether this matters to you or not is your choice. There are really only two practical outcomes. A consumer of the microdata of your page can either create a WebPage item, or not create WebPage item. It is highly unlikely that a consumer would refuse to create a WebPage item because of the presence an additional out-of-schema itemprop, where it would have created the WebPage item otherwise.
And that's ultimately what validity is all about. It's there to establish a common language that producers and consumers of documents both understand. Providing that consumers understand the information that producers provide, technical violations of the validity rules of any particular specification are of little consequence.