Disallow setting invalid values for registered properties via CSSOM

This patch rejects setting invalid values for registered properties via
the CSSStyleDeclaration::setProperty function. The behaviour which we
decided on is that we will retain any values set prior to registration
and compute them as if they were the unset keyword if they are invalid
after registration, but disallow setting such values after registration.

CSS Typed OM is not yet hooked up aside from the filtered computed map
for custom paint, so we don't yet make any changes there.

BUG=641877

Review-Url: https://codereview.chromium.org/2607403002
Cr-Commit-Position: refs/heads/master@{#441313}
25 files changed