Cleanup RenderTreeAsText

Cleanup RenderTreeAsText as well as SVGRenderTreeAsText:

- fix outdated comment
- remove unused methods
- share stream insertion operator for Color

BUG=416453

Review URL: https://codereview.chromium.org/595603002

git-svn-id: svn://svn.chromium.org/blink/trunk@182452 bbb929c8-8fbe-4397-9dbb-9b2b20218538