Make image-zoom-to-25.html not depend on sub-pixel precision

Use Math.round on image.clientWidth/clientHeight since those
could have sub-pixel precision after blink r173144. (round's
sufficient for this test, sub-pixel precision or not).

TBR=eae@chromium.org
BUG=369376

Review URL: https://codereview.chromium.org/269733002

git-svn-id: svn://svn.chromium.org/blink/trunk@173171 bbb929c8-8fbe-4397-9dbb-9b2b20218538