Don't try to extend non-directional selection if selection has been gone

This patch changes Shift+LeftButton+Down to extend non-directional selection not to crash by checking whether selectable DOM Node |Position| is null or not-null.

The root cause of crash in issue 324975 is getting |Range| object for null |Position|, which isn't allowed, in |textDistance(Position position, Position reference)|.

Note: At this time, non-directional selection is enabled only for MacOSX.

BUG=324975
TEST=LayoutTests/fast/events/shift-click-user-select-none.html

Review URL: https://codereview.chromium.org/177003026

git-svn-id: svn://svn.chromium.org/blink/trunk@168106 bbb929c8-8fbe-4397-9dbb-9b2b20218538