Let Selection.deleteFromDocument not delete a character when the selection is a caret.

Selection.deleteFromDocument delete a character when the selection is a caret.
However, current standard says that Selection.deleteFromDocument does nothing when the selection is a caret:
https://dvcs.w3.org/hg/editing/raw-file/tip/editing.html#dom-selection-deletefromdocument

Both IE10 and FireFox seem following the spec.

BUG=366498, 362110

Review URL: https://codereview.chromium.org/255453003

git-svn-id: svn://svn.chromium.org/blink/trunk@172511 bbb929c8-8fbe-4397-9dbb-9b2b20218538