[svg] Make use-element-web-animations.html actually animate (#62697)

The test never ran. Two separate problems, both silent.

font-size is not a legal identifier in a JavaScript object literal, so
the script was a SyntaxError and no animation was ever started. Quoting
the key is not enough: a dashed keyframe key is not a valid keyframe
property name either, and it is dropped in Chrome 155, Firefox 158 and
Safari 26.4 and 27.0, all measured. That is already pinned by
web-animations/interfaces/KeyframeEffect/processing-a-keyframes-argument-001.html,
where 'font-size' sits in gNonAnimatableProps commented "Supported
property that uses dashes", and that test is 73/73 in all four engines.
So the key has to be fontSize.

With that fixed the animation ran, but in Safari and Firefox it slid
linearly from 120px to 80px, because steps(2, start) was on the empty
first keyframe. Per the effect value algorithm, keyframes with no value
for the target property are removed before interval selection, an
implicit keyframe with no easing is synthesized at offset 0 in their
place, and the transformed distance uses the easing of the first
interval endpoint. So the author's easing could never reach font-size,
and the rendering depended on when the screenshot was taken: it matched
the reference only at exactly currentTime 50000.

Moving the easing into the effect options makes the value flat across
the whole first half in all three engines, so the timing dependence is
gone everywhere. It also matches the sibling use-element-animations.html,
where steps(2, start) is the animation-level timing function.

The empty first keyframe is kept on purpose. The point of the test is
that each element starts from its own computed font-size, 120px for the
original and 40px for the use copy, so the start value must not be
written down.

Measured after the change, against use-element-transitions-ref.html: the
original text animates to 100px in all three engines and its bounding
box matches the reference, while the use copy stays at its inherited
40px. So the test now fails because the shadow tree instance gets no
mirrored animation, which needs ShadowAnimation and is implemented
nowhere, rather than because the script did not parse. It is still
expected to fail in every engine, and no expectation metadata changes.