Fix handling of currentColor in svg animations

The current implementation fails to correctly handle currentColor in
some cases. This patch makes the code check the valid flag on Color
(which should refer to currentColor in these scenarios) instead of
using the propertyValueType mechanism. The fixed behavior matches
what Firefox currently does.

R=pdr
BUG=331711

Review URL: https://codereview.chromium.org/123453005

git-svn-id: svn://svn.chromium.org/blink/trunk@164726 bbb929c8-8fbe-4397-9dbb-9b2b20218538