Assert that Length::blend is called with specified values

The checks in Length::blend for non-specified values were for the old
CSS animations/transitions code paths which have now been removed. We
should no longer get into Length::blend with non-specified values.

BUG=

Review URL: https://codereview.chromium.org/234413004

git-svn-id: svn://svn.chromium.org/blink/trunk@171426 bbb929c8-8fbe-4397-9dbb-9b2b20218538