Fixed Layers panel for devices with window.devicePixelRation != 1.

Review URL: https://codereview.chromium.org/236203009

Review URL: https://codereview.chromium.org/236203009

git-svn-id: svn://svn.chromium.org/blink/trunk@171444 bbb929c8-8fbe-4397-9dbb-9b2b20218538