Fix test should-not-repaint-composited-descendants.html

When the test was added (https://codereview.chromium.org/234413002/),
it was supposed to test the non-layout path on some css property
changes, but accidentally it included a 'width' change which actually
triggers layout.

Remove 'width' property to test the non-layout path.

Review URL: https://codereview.chromium.org/239643003

git-svn-id: svn://svn.chromium.org/blink/trunk@171717 bbb929c8-8fbe-4397-9dbb-9b2b20218538