Add early-out to RenderSVG{Block,ModelObject}::repaintTreeAfterLayout

Follow-up to https://codereview.chromium.org/264963004/ and
https://codereview.chromium.org/260963012/.

BUG=369486, 320139

Review URL: https://codereview.chromium.org/263393003

git-svn-id: svn://svn.chromium.org/blink/trunk@173415 bbb929c8-8fbe-4397-9dbb-9b2b20218538