Output Element defaultValue should be based on textContent

Default value based on textContent if flag is in mode default.

R=tkent, keishi1
BUG=345788
TEST=Check default value based on textContent.

Review URL: https://codereview.chromium.org/178253006

git-svn-id: svn://svn.chromium.org/blink/trunk@168703 bbb929c8-8fbe-4397-9dbb-9b2b20218538