[SVG] Refactoring accumulate logic for animateMotion

made some changes to simplify the logic for accumulation

R=pdr@chromium.org, schenney@chromium.org, dschulze@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/19500005

git-svn-id: svn://svn.chromium.org/blink/trunk@154444 bbb929c8-8fbe-4397-9dbb-9b2b20218538