Revert "USERSELECT_ALL flag is removed and the feature is enabled."

r154244 caused a regression breaking text selection in Yahoo Mail.

This reverts commit 8561674657274e9edc16f13a75b0940b22868281.

R=ojan@chromium.org
BUG=262136

Review URL: https://chromiumcodereview.appspot.com/20002006

git-svn-id: svn://svn.chromium.org/blink/trunk@154728 bbb929c8-8fbe-4397-9dbb-9b2b20218538