DevTools: follor up to r170181, fixing inspector/layers/layer-scroll-rects-get.html.
TBR=vsevik

Review URL: https://codereview.chromium.org/214693006

git-svn-id: svn://svn.chromium.org/blink/trunk@170191 bbb929c8-8fbe-4397-9dbb-9b2b20218538