Improve tests, related to zooming ImageDocuments.

Previously, these tests relied on resize events, which is not necessary,
since zooms are synchronous. I compacted the two tests into one and
made them a bit more value-resilient -- instead of just printing values,
they actually tests for size equality.

BUG=307570
R=eseidel@chromium.org, tony

Review URL: https://codereview.chromium.org/29543002

git-svn-id: svn://svn.chromium.org/blink/trunk@159954 bbb929c8-8fbe-4397-9dbb-9b2b20218538