Try to deflake svg dynamic updates tests

These tests are flakily failing due to slight image differences.  It's
possible that some incremental layout is laying out the scaled image
weirdly and rounding differently than another path.  Attempt to patch
over this in the test by using a square (400x400) image.

R=pdr@chromium.org
BUG=374936

Review URL: https://codereview.chromium.org/292123006

git-svn-id: svn://svn.chromium.org/blink/trunk@174525 bbb929c8-8fbe-4397-9dbb-9b2b20218538