[CSS Shapes] Can't select content within the area of the floating box when clip-path is applied

According to http://www.w3.org/TR/css-masking-1/, the clipped part of the element should not have pointer event dispatching. So add this logic to RenderBlock::nodeAtPoint. This fixes the bug as it disallows selection on any clipped parts.

BUG=364153

Review URL: https://codereview.chromium.org/240423005

git-svn-id: svn://svn.chromium.org/blink/trunk@172619 bbb929c8-8fbe-4397-9dbb-9b2b20218538