Remove all but one of the PaintLayerFlags.

Only PaintLayerPaintingOverlayScrollbars is actually ever set
to a different value in different codepaths.

-PaintLayerHaveTransparency can just be replaced with
isTransparent() calls.
-PaintLayerPaintingCompositingScrollingPhase is unused.
-PaintLayerPaintingCompositingAllPhases is always set,
which means that the other three compositing ones are
also always set.

The third bullet more clearly falls out when you see that
the only caller of paintLayerContents is
paintLayerContentsAndReflection, which sets
PaintLayerPaintingCompositingAllPhases on the paint flags.
Get rid of paintLayerContentsAndReflection since we no
longer have reflections.

The rest of the changes are all just propagating the now
always false or true booleans in paintLayerContents.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/778883002