If an element has unicode-bidi: plaintext and no strong directional characters. By default it should be LTR.

BUG=277687

Review URL: https://chromiumcodereview.appspot.com/23112019

git-svn-id: svn://svn.chromium.org/blink/trunk@156664 bbb929c8-8fbe-4397-9dbb-9b2b20218538