Fix for broken undo functionality

When delete(character granularity) is performed after few text insertions,
this delete command is added to already open typing command. This was
resulting in wrong 'undo' behaviors.

This can be fixed by keeping continuous delete operations alone as a group.
Hence whenever a delete command follows some other typing command say text
insertion, we should open a new typing command.

This fixes many erroneous behaviors related to undo operation.

TEST=LayoutTests/editing/undo/undo-insert-delete.html

BUG=319659

Review URL: https://codereview.chromium.org/65043014

git-svn-id: svn://svn.chromium.org/blink/trunk@162784 bbb929c8-8fbe-4397-9dbb-9b2b20218538