Remove the ENABLE_DETAILS_ELEMENT command-line flag.

This mechanical patch should have no web-visible impact; Chromium has enabled
the <details> element since M12 or so.

Review URL: https://chromiumcodereview.appspot.com/14226002

git-svn-id: svn://svn.chromium.org/blink/trunk@148330 bbb929c8-8fbe-4397-9dbb-9b2b20218538