Fixed layer bounds. 

Using WebLayer.bounds instead of GraphicsLayer.size in InspectorLayerTreeAgent. Fixes empty layers in DevTools Layers panel.

Review URL: https://codereview.chromium.org/169673002

Review URL: https://codereview.chromium.org/169673002

git-svn-id: svn://svn.chromium.org/blink/trunk@167517 bbb929c8-8fbe-4397-9dbb-9b2b20218538