Compute correct repaint rect for decorated RenderSVGRoots

The current implementation of clippedOverflowRectForRepaint() uses the
generic repaint-rect calculations in SVGRenderSupport. Those in turn make
use of repaintRectInLocalCoordinates(), which for RenderSVGRoot is the
union of the painted children (w/ some expansion). If there're no children,
or they do not fill the entire content box, then a repaint would not
repaint the correct parts.
Fix by calculating the union of the border-box and the SVG content
when the SVG root is decorated (has background/border/etc.)

BUG=354305

Review URL: https://codereview.chromium.org/211193002

git-svn-id: svn://svn.chromium.org/blink/trunk@170890 bbb929c8-8fbe-4397-9dbb-9b2b20218538