Only call positionOverflowControls() if the layer is actually scrollable

We spend 16% of the time in RenderLayer::styleChanged updating the scroll
bar controls even when layers are not scrollable in the transform-benchmark.html
which is also 25% of the time inside 
CompositedLayerMapping::updateGraphicsLayerGeometry().

This patch makes us only call it when we really can scroll.

In the Silk infinite scrolling benchmark, this CL makes Document::recalcStyle
13.8% faster.  It also makes RenderLayerCompositor::updateLayerTreeGeometry
12.8% faster.

BUG=335703

Review URL: https://codereview.chromium.org/147083006

git-svn-id: svn://svn.chromium.org/blink/trunk@165899 bbb929c8-8fbe-4397-9dbb-9b2b20218538