Clean up layout tests in LayoutTests/shadow-dom/

- Remove "v1-" prefix.
- Clean up styles
- Use makeTestTree() as much as possible

Clean up is not finished. Other clean up CLs will come later.

BUG=531990

Review-Url: https://codereview.chromium.org/2053503003
Cr-Commit-Position: refs/heads/master@{#398827}
40 files changed