Adds an #ifdef to the DCLayerTree implementation that allows
selecting whether to use the virtual surface API (enabled by
default) or the regular DC surface API.

For now, this is a compile-time switch. As a follow up to this,
we will support both options at runtime (for example, using the
regular surface API for surfaces that have holes or translucency).

Differential Revision: https://phabricator.services.mozilla.com/D58870

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/1d212c2ddb76f5d54fd884e8e08001fc8475e13c