Fix transition event property name for transitions on shorthands

This patch fixes the propertyName value of the transition event to
correctly return the transitioned property in the case of shorthands.
This was initiall broken by crrev.com/212483003. Note that due to the
transform unprefixing changes, we'll only fire a single event when
-webkit-transform-origin is set as the transition property.

This fixes the w3c transition tests events-001 and events-005.

BUG=375098

Review URL: https://codereview.chromium.org/297663002

git-svn-id: svn://svn.chromium.org/blink/trunk@174357 bbb929c8-8fbe-4397-9dbb-9b2b20218538