Clean up isValidKeywordPropertyAndValue

This patch cleans up the isValidKeywordPropertyAndValue function to be
slightly less error-prone. The current pattern here is a bit silly, as
accidentally leaving out the break will cause a switch fall-through and
may be hard to notice. The scroll-behavior property was incorrectly
falling through here (causing an assertion error in the StyleBuilder,
which I've fixed. To avoid future similar issues, I've rewrote the
function to simply return a boolean expression for each case.

BUG=374530

Review URL: https://codereview.chromium.org/289253003

git-svn-id: svn://svn.chromium.org/blink/trunk@174505 bbb929c8-8fbe-4397-9dbb-9b2b20218538