Fixed pseudo-element ::selection for text node in div.

R=tkent@chromium.org
BUG=365784
TEST=fast/selectors/shadow-host-div-with-text.html

Review URL: https://codereview.chromium.org/257463003

git-svn-id: svn://svn.chromium.org/blink/trunk@173320 bbb929c8-8fbe-4397-9dbb-9b2b20218538