Clean up some booleans in RenderLayer::paintLayerContents

The if-statement conditions have become unwieldy and unintuitive
in RenderLayer::PaintLayerContents().  Furthermore, the
nice "phases of painting" get tangled because of the way that
composited paint phases communicate to the RenderLayer.  This
patch is just a baby step to cleaning that up, by pulling the
if-statement conditions into temporary booleans.

BUG=261608
R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/19677005

git-svn-id: svn://svn.chromium.org/blink/trunk@155009 bbb929c8-8fbe-4397-9dbb-9b2b20218538