This is a completion of Eric's WIP patch:
https://codereview.chromium.org/1179663005/

Low level support for creating a paragraph that contains
runs of styled text. The styles may be nested.

The Paragraph and RenderParagraph classes have been
replaced by Inline and RenderInline. Styled text is defined
with a tree of InlineText and InlineStyle objects.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1177833012.