WebKit's (and I believe in Gecko's) implementation "consume user activation" returns a boolean.
This has nice qualities in that one can do:
if (consume user activation of W), then ...
Implying that it had transient activation in order to consume the activation.
This could be useful in https://github.com/w3c/webdriver/pull/1695/files ... where it would be good to actually know if consuming the activation succeeds (or not).