Fix a crash for extractSmartClipData

Position::document() returns null if -webkit-user-select is set to none.
This CL prevents the crash in this case. Also adds a test to make sure
that the crash does not happen and clip text and clip html are empty.

TEST=passed WebViewTest
BUG=402371

Review URL: https://codereview.chromium.org/455133003

git-svn-id: svn://svn.chromium.org/blink/trunk@180701 bbb929c8-8fbe-4397-9dbb-9b2b20218538