First step at getting rid of anonymous blocks and continuations.

-Add RenderParagraph and display:paragraph. This is the only
render type that's allowed to contain inlines or text.
-If you put text nodes directly in a non-paragraph, wrap them
in an anonymous paragraph. This may not be the place we want
to end up, but it's a good stopgap to make it so we don't
crash in this case.
-Make StyleAdjuster force that non-paragraph blocks only contain
RenderBlock subclasses and that paragraphs and inlines only contain
inlines.
-Considerably simplify addChildIgnoringAnonymousColumnBlocks
now that we only create anonymous blocks for the case of
text nodes in non-paragraphs. Also get rid of the behavior
where we try to group multiple nodes into a single
anonymous block.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/729693003