Fix bugs in sheriff-o-matic's pixel zoomer.

-x/y were swapped. Create a mock canvas context so we actually
test this code path.
-The zoomer was below the test list. z-index:1 to put it above.

NOTRY=true

Review URL: https://codereview.chromium.org/423703002

git-svn-id: svn://svn.chromium.org/blink/trunk@179073 bbb929c8-8fbe-4397-9dbb-9b2b20218538