DevTools: show actual layer transforms on the 3D view

We still default to translateZ(20px) if there's no transform, but
override it with actual transform matrix if layer is transformed.
Also use appropriate scale for Z axis and extract layer thickness
and spacing into a stylesheet compensated for Z-axis scale.

BUG=

Review URL: https://chromiumcodereview.appspot.com/22804003

git-svn-id: svn://svn.chromium.org/blink/trunk@156150 bbb929c8-8fbe-4397-9dbb-9b2b20218538