Fix computation of local-to-border-box transform for RenderSVGRoot

Use isZero() instead of isEmpty() to correctly handle cases where only
one of the "dimensions" are non-zero.

BUG=377362

Review URL: https://codereview.chromium.org/302433009

git-svn-id: svn://svn.chromium.org/blink/trunk@174855 bbb929c8-8fbe-4397-9dbb-9b2b20218538