Avoid useless initialization in ScrollAnimatorNone::PerAxisData::animateScroll()

Avoid useless initialization in ScrollAnimatorNone::PerAxisData::animateScroll().
The value initially assigned to newPosition was always overwritten. Therefore,
the initial assignment can be omitted. This CL moves the initialization code to a
new inline function and initializes directly the newPosition variable with the
result of this new function.

The clang static code analizer was reporting this as a dead initialization.

R=abarth@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/244253002

git-svn-id: svn://svn.chromium.org/blink/trunk@172001 bbb929c8-8fbe-4397-9dbb-9b2b20218538