Allow determining if an SVG value has been specified

When checking for the "presence" of a value (if a value has been
specified - either via an attribute or via an animation), using
hasAttribute is not sufficient since if misses if the value is being
animated.

Extend the property interfaces to allow providing an answer to this query.

R=kouhei@chromium.org
TEST=svg/animations/no-attr-*.svg
BUG=330191

Review URL: https://codereview.chromium.org/141793002

git-svn-id: svn://svn.chromium.org/blink/trunk@165537 bbb929c8-8fbe-4397-9dbb-9b2b20218538