Fix preferred width calculation regression for rtl with word-break: break-words

Blink r165125 regressed how we calculate preferred width in bidi contexts. It
was meant to cause preferred width calculation to split "words" at text direction
boundaries, but missed that BidiCharacterRun includes its end (e.g. a run that
starts at 0 and ends at 0 is actually 1 character long).

I'm taking this as a lesson that RenderText::computePreferredLogicalWidths
needs to be refactored.

BUG=336497

Review URL: https://codereview.chromium.org/153473006

git-svn-id: svn://svn.chromium.org/blink/trunk@166910 bbb929c8-8fbe-4397-9dbb-9b2b20218538