[css-properties-values-api] Include initial values in computedStyleMap.

Registered properties with initial values are currently missing from the
computedStyleMap, because such properties are not explicitly stored on
individual computed styles.

To fix this, do an extra pass over the PropertyRegistrations to "fill in
blanks" with initial values.

R=chrishtr@chromium.org

Bug: 641877
Change-Id: If36cb3ef3d419ff547e8b4073cdd9ec298c539a0
Reviewed-on: https://chromium-review.googlesource.com/c/1264575
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598243}
4 files changed