Fix handling of bidirectional override

Fix handling of BDO (bidirectional override) elements by explicitly
checking for bidi override in RenderText and using the specified
direction instead of resolving based on content.

R=leviw@chromium.org
BUG=352160
TEST=fast/text/international/bdo-bidi-width.html

Review URL: https://codereview.chromium.org/197883020

git-svn-id: svn://svn.chromium.org/blink/trunk@169461 bbb929c8-8fbe-4397-9dbb-9b2b20218538