Take directionality into account for img alt text

Change RenderFlowBox::constructTextRunInternal to use a BidiResolver
to determine directionality for the case where a run is constructed
from a String (as oppsed to a RenderText object) and the directional
Override bit has not been set.

R=eseidel@chromium.org
BUG=326921
TEST=fast/text/bidi-img-alt-text.html

Review URL: https://codereview.chromium.org/101973006

git-svn-id: svn://svn.chromium.org/blink/trunk@163930 bbb929c8-8fbe-4397-9dbb-9b2b20218538