Fix computation of animated transform w/ different from/to transform types

Calculating the animated value between two transforms of differing types
would give random results (and ASSERT in debug.)
Fix by requiring the effective 'from' value to have the same type as the
'to' value. If the types are not the same, then use a "zero transform" as
the 'from' value.

Also adjust the behavior of animateTransform additive to-animations in
this case by clearing the list in these cases too (matching Presto and
Gecko in this particular case.)

BUG=231409

Review URL: https://codereview.chromium.org/330813002

git-svn-id: svn://svn.chromium.org/blink/trunk@176193 bbb929c8-8fbe-4397-9dbb-9b2b20218538