Properly test unset behavior of invalid registered property values

This patch fixes registered-property-cssom.html to properly test that
invalid registered property values are computed as unset. The test was
missing inherited values so the unset value was always resolving to the
initial value.

BUG=641877

Review-Url: https://codereview.chromium.org/2621553002
Cr-Commit-Position: refs/heads/master@{#442197}
1 file changed