Right align alt-text for RTL images

Change the rendering of image alt text to render the text right aligned
for images with RTL direction.

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

Review URL: https://codereview.chromium.org/140823018

git-svn-id: svn://svn.chromium.org/blink/trunk@165623 bbb929c8-8fbe-4397-9dbb-9b2b20218538