Fix RenderObject::assertSubtreeIsLaidOut().

For a fleeting moment I thought I had fixed all the SVG layout issues :)

R=jchaffraix@chromium.org,ojan@chromium.org,eseidel@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23750006

git-svn-id: svn://svn.chromium.org/blink/trunk@157905 bbb929c8-8fbe-4397-9dbb-9b2b20218538