[FastTextAutosizer] Fix incorrect override for RenderSVGShape::isEmpty

This patch fixes a crash on xe.com where the text autosizer would call
isEmpty() on a RenderObject and crash. The issue is that RenderSVGShape
overrides RenderObject::isEmpty and implements it differently:
RenderObject::isEmpty() - true if the RenderObject has children
RenderSVGShape::isEmpty() - true if the bounding box is empty

I've refactored RenderSVGShape::isEmpty() as isShapeEmpty() to both
clarify the override and fix a crash.

BUG=302005
NOTRY=true

Review URL: https://codereview.chromium.org/185243002

git-svn-id: svn://svn.chromium.org/blink/trunk@168252 bbb929c8-8fbe-4397-9dbb-9b2b20218538