Change text-based-repaint to dump only graphics layer tree

In order to simplify changing svg repaint tests that currently don't
dump any text to dumping text, modify current text-based-repaint clients
to only dump the graphics layer tree.

This changes some images (where the text output in the page had
previously affected overflow and layer sizes), but mostly trims the text
output to no longer include the page content and only include the
graphics layer tree.

BUG=369609

Review URL: https://codereview.chromium.org/264973017

git-svn-id: svn://svn.chromium.org/blink/trunk@173452 bbb929c8-8fbe-4397-9dbb-9b2b20218538