DevTools: in 3d layer view, use Z-axis spacing of layers even if these are transformed

We used to to translateZ(layerSpacing) only for non-transformed layers. Now we do it for
all layers.

BUG=

Review URL: https://chromiumcodereview.appspot.com/22852022

git-svn-id: svn://svn.chromium.org/blink/trunk@156631 bbb929c8-8fbe-4397-9dbb-9b2b20218538