Have layerTreeAsText dump subframes.

In a future test, I'll need to dump the outer and inner frames
in the same frame (as in, BeginMainFrame). But, since
layerTreeAsText forces another compositing update, it defeats
my test. In general, I don't see any reason not to
dump the layer trees of subframes.

I was a little unsure what to do with subframes that aren't
composited. For now, I decided not to dump them as they
added noise to some tests.

Review URL: https://codereview.chromium.org/274203002

git-svn-id: svn://svn.chromium.org/blink/trunk@173782 bbb929c8-8fbe-4397-9dbb-9b2b20218538