Convert bidi-img-alt-text to pixel test

Trying to use a ref test for font rendering turned out to be an
excessive in futility, fall back on a pixel test.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/122243003

git-svn-id: svn://svn.chromium.org/blink/trunk@164461 bbb929c8-8fbe-4397-9dbb-9b2b20218538