Add a number of tests for layer squashing

This CL converts a number of manual tests from crbug.com/315321 into
LayoutTests. In this CL, I've converted a number of simple cases from
comment 2 on that issue. In a future CL, I'll convert more tests.

BUG=315321

Review URL: https://codereview.chromium.org/137143009

git-svn-id: svn://svn.chromium.org/blink/trunk@166865 bbb929c8-8fbe-4397-9dbb-9b2b20218538