[SVG2] Svg elements that overflow the viewport should also be hittested.

When we have an svg that doesn't clip to its viewport (aka
overflow:visible), and there is some overflowing content, we
need to consider the overflowing elements when hit testing.
Otherwise it becomes impossible to interact with svg
elements outside the viewport rect.

BUG=401443

Review URL: https://codereview.chromium.org/444223005

git-svn-id: svn://svn.chromium.org/blink/trunk@179806 bbb929c8-8fbe-4397-9dbb-9b2b20218538