Minor cleanup to RenderLayerScrollableArea::setNeedsCompositedScrolling.

There's no reason for this function to return a bool. One caller
cares about it and the other doesn't. It's should be
the caller's job to track whether the bool changed.

Review URL: https://codereview.chromium.org/234423007

git-svn-id: svn://svn.chromium.org/blink/trunk@171371 bbb929c8-8fbe-4397-9dbb-9b2b20218538