Validate CompositedSelectionBounds before forwarding

As |RenderedPosition::positionInGraphicsLayerBacking| makes no
guarantees that it will find an appropriate layer backing, the caller
should not assume success. Check the result explicitly when updating
the composited bounds, ensuring their validity before forwarding to the
client. Also support forwrding of an empty (zero height) bound.

BUG=417306

Review URL: https://codereview.chromium.org/612323008

git-svn-id: svn://svn.chromium.org/blink/trunk@183366 bbb929c8-8fbe-4397-9dbb-9b2b20218538