Init BidiResolver with Neutral directionality in RenderText

Set the initial directionality to Neutral in
RenderText::computePreferredLogicalWidths. This ensures that leading and
trailing SCRIPT_COMMON characters gets the correct directionality and
thus the correct width.

TEST=fast/text/international/rtl-space-in-ltr-element.html
BUG=333004
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/185123002

git-svn-id: svn://svn.chromium.org/blink/trunk@168339 bbb929c8-8fbe-4397-9dbb-9b2b20218538