Allow hitting the SVG root when its display-type is 'block'

The 'HitTestChildBlockBackground' hit test action needs to be considered
as well when detecting hits on the SVG root container itself.

BUG=379299

Review URL: https://codereview.chromium.org/318663002

git-svn-id: svn://svn.chromium.org/blink/trunk@175528 bbb929c8-8fbe-4397-9dbb-9b2b20218538