Disable image filtering in layout test result zoomer

Draw the zoomed-in layout test result images without filtering to make
it easier to see which parts have changed.

Before: https://drive.google.com/file/d/0ByyxMXB38gLDbWhIYnZPNVRDNXc/edit?usp=sharing

After: https://drive.google.com/file/d/0ByyxMXB38gLDaC1aV0VzZUtGZ28/edit?usp=sharing

Review URL: https://codereview.chromium.org/158083004

git-svn-id: svn://svn.chromium.org/blink/trunk@167118 bbb929c8-8fbe-4397-9dbb-9b2b20218538