I'm not 100% what this was for, but looking at Blink,
it appears that HitTestChildBlockBackground being different
from HitTestBlockBackground had something to do with
multi-column/regions, which we don't have. I believe
this patch doesn't change any behavior.

Also added to the elementFromPoint test in order to
get more test coverage of the hitTesting code.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/924263002