Add support for multiple text decorations with same line positioning
Add support for multiple propagated text decorations with the same line
positioning, as defined in
http://www.w3.org/TR/css-text-decor-3/.
Before this, elements with multiple text decorations but the same line
positioning (e.g. two underlines) would only paint the 'topmost' one.
With this patch, both lines are painted.
This is important for CSS Text Decorations Module 3, where two text
decorations can have the same line position but different styles (e.g.
wavy and solid), and they should be painted on top of eachother.
This minimally affects some tests where two text decorations with the
same line positioning were present but only one was being painted.
Also cleaned up the existing logic for text decorations to only inherit
through StyleAdjuster, removing the need to traverse up the tree to
resolve colors for inherited text decorations.
BUG=
462142