ce: Don't set backgroundColor transparent for every node in debug mode

This only affects debug mode, but things look broken when the
background of every single node is set to transparent (e.g. a
<pre> looks weird with a white instead of grey background).

It also leaves the DOM dirty full of inline styles.

Though setting a grey background isn't guaranteed to be visible
either, and all of these redraws and stuff really slow things
down (we should perhaps only start doing these after the initial
document is painted).. the least we can do is undo it and not
leave it there.

Change-Id: I9abfd46765914828ad8618748be5716a8c6b185c
Merge "ce: Don't set backgroundColor transparent for every node in debug mode"