Handle surrogates in BidiResolver::determineParagraphDirectionality

TEST=unittest:BidiResolver.ParagraphDirectionSurrogates
BUG=322051

Review URL: https://codereview.chromium.org/83443002

git-svn-id: svn://svn.chromium.org/blink/trunk@162915 bbb929c8-8fbe-4397-9dbb-9b2b20218538