SMIL animation which fill mode equal 'freeze' will use the specified from/to value,
current servo use SvgLengthOrPercentageOrNumber, so animated value when an animation
is finished will be LengthOrPercentage value.

i.e.
 If we animate between '20' and '10px'(<animate from='20' to='10px' fill='freeze'>):
    0%   -> 20
    50%  -> 15
    100% -> 10px

 The result of computed value contains unit since smil use specified 'to' value.

SMIL implementation of gecko side will call Servo_ParseProperty when specified
fill='freeze' and animation finished.

So this PR add animated value for stroke-* property. This animated value
contains the calc() value, but gecko side implementation doesn't support calc().
For detail, see https://bugzilla.mozilla.org/show_bug.cgi?id=1386967.