Remove Ununsed Input Variable from ScrollAnimator NotifyPosChange

NotifyPositionChange pass on stored m_currentPos, and didn't use its
input variable. Removing its input variable from signature to reflect
this.

BUG=

Review URL: https://codereview.chromium.org/212063004

git-svn-id: svn://svn.chromium.org/blink/trunk@170499 bbb929c8-8fbe-4397-9dbb-9b2b20218538