[SVG] animateMotion accumulate doesn't work properly with rotate: auto / auto-reverse

We need not accumulate the angle for animation as we have a method to get normal angle at a given length on the path.
So, only position needs to be accumulated. Also made some changes to simplify the logic for accumulation.

R=pdr@chromium.org, schenney@chromium.org, dschulze@chromium.org
BUG=256656

Review URL: https://chromiumcodereview.appspot.com/18436004

git-svn-id: svn://svn.chromium.org/blink/trunk@154375 bbb929c8-8fbe-4397-9dbb-9b2b20218538