Is there the theoretical possibility that Google could turn around and use the patent offensively? Rescind their grant? Or are we all now guaranteed safe to use it?
The license includes a "perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable (except as stated in this section) patent license." - http://www.webmproject.org/license/software/
You'll have to ask a lawyer whether it's truly irrevocable, but clearly that's the intention.
