Make CompositeEditCommand::prepareWhitespaceAtPositionForSplit() works at deleting SVG element

CompositeEditCommand::prepareWhitespaceAtPositionForSplit() replaces collapsible whitespaces with non-breaking space around split position.

This patch changes it to work when split position is end of text node, e.g. VisualPosition.downstream().deprecatedNode() != VisualPostion.deprecateNode().

BUG=252093
TEST=LayoutTests/editing/deleting/delete-svg-001.html
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/17483003

git-svn-id: svn://svn.chromium.org/blink/trunk@158002 bbb929c8-8fbe-4397-9dbb-9b2b20218538