Delete some almost-dead code in the compositor (and elsewhere) for plumbing paint
invalidation rectangles.

Instead, change the tests that use this code path to use the more common one that is similar.
Also, remove an unused method on Internals for plumbing paint invalidation rectangles as text.

BUG=

Review URL: https://codereview.chromium.org/349223005

git-svn-id: svn://svn.chromium.org/blink/trunk@176798 bbb929c8-8fbe-4397-9dbb-9b2b20218538