Filters blur(), invert(), etc. can omit argument.
Computed/specified style serialization is a little tricky w.r.t the shortest
serialization principle. Ideally we should serialize `invert(1)` to `invert()`,
but that will be a breaking change, so we always serialize them with an
argument.
Note, Blink/WetKit treat specified (but not computed) style serialization
differently when the specified one is originally without argument. Our
current behavior is the same as pre-Chromium Edge.
Differential Revision: https://phabricator.services.mozilla.com/D31720
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extra : moz-landing-system : lando
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