Git rid of default direction for constructTextRun

Change RenderBlockFlow::constructTextRun to get remove the default
value for textDirection, requiring that call sites either specify the
desired direction or use the string based version that resolves the
directionality based on content.

BUG=333004
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/148233004

git-svn-id: svn://svn.chromium.org/blink/trunk@165949 bbb929c8-8fbe-4397-9dbb-9b2b20218538