[css-properties-values-api] Reify values from styleMap.get.

This CL gives return values from element.styleMap.get the correct type
according to the current PropertyRegistration.

The expected behavior is exactly the same as for attributeStyleMap, so
those tests have been adapted to run for both API entry points.

Note: PrepopulatedComputedStylePropertyMap is available in worklets,
where the ExecutionContext is not a Document, but we don't need to
reify values in this case, because already-reified values are "pushed"
rather than "pulled". (See ::UpdateStyle).

Note: Exit ParseIfRegistered before registration lookup. This avoids
the lookup if we already know that we won't need it (which is the case
for [Prepopulated]ComputedStylePropertyMap).

Bug: 641877
Change-Id: Id54a22079c88368efccc372d01252fa3e7734d10
Reviewed-on: https://chromium-review.googlesource.com/1201302
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588840}
7 files changed