Tweak the descriptions of the various types of layout tests.

This CL adjusts the work in r434842 to better describe the
differences between pixel tests and render tree tests and when
you want one or the other. It also gets rid of the "DumpRenderTree"
terminology since we don't really use that any more.

R=pwnall@chromium.org, me@gsnedders.com
BUG=665494

Review-Url: https://codereview.chromium.org/2547463003
Cr-Commit-Position: refs/heads/master@{#443810}
1 file changed