Correct the elapsedTime calculation in SVG animations

As per the SVG standard, SVG document on receiving the
pauseAnimations() should also pause the animation clock corresponding
to the SVG document. Whereas current implmentation also takes into
account the accumulated pause time which is not required.
So replacing the m_accumulatedPauseTime with m_accumulatedActiveTime
and other corresponding calculations.
Added the layout Test.

BUG=254067

Review URL: https://chromiumcodereview.appspot.com/18374004

git-svn-id: svn://svn.chromium.org/blink/trunk@154603 bbb929c8-8fbe-4397-9dbb-9b2b20218538