Make invisible-descendants-should-not-affect-opt-in dump its trees.

The test currently tests if layers were promoted by testing if
compositing was enabled. When ForceCompositingMode is on, this test
is no longer valid. Instead dump the layer tree so we can see if
layers were promoted or not.

R=enne@chromium.org, vollick@chromium.org
BUG=361729

Review URL: https://codereview.chromium.org/239303002

git-svn-id: svn://svn.chromium.org/blink/trunk@171673 bbb929c8-8fbe-4397-9dbb-9b2b20218538