Prepare for enabling squashing

When the compositor change to enable layer squashing lands, many layout
tests will start failing due to benign text differences (since the
GraphicsLayer tree topologies will be different). This CL temporarily
marks these tests as flaky so the compositor change can land.

Rebaselines are requested here: https://codereview.chromium.org/238293011
Compositor change is here: https://codereview.chromium.org/240653005/

NOTRY=true
BUG=261605

Review URL: https://codereview.chromium.org/239143011

git-svn-id: svn://svn.chromium.org/blink/trunk@171872 bbb929c8-8fbe-4397-9dbb-9b2b20218538