REGRESSION(r156139) Fix placeholder handling at end of editable root

This patch changes placeholder handling at end of editable root in DeleteSelectionCommand::doApply() to insert placeholder in allow below conditions are satisfied after selection deleted:
 - there is a placeholder.
 - there is line break before deleted selection.
 - line break wasn't at end of deleted selection.

editing/deleting/delete-character-002.html
 - Test cases for TEXTAREA

editing/deleting/delete-character-003.html
 - Test cases for HTML editing

editing/deleting/delete-character-004.html
 - Moved a test case to delete-character-003.html

BUG=264986
TEST=LayoutTests/editing/deleting/delete-character-002.html, LayoutTests/editing/deleting/delete-character-003.html

Review URL: https://chromiumcodereview.appspot.com/23857002

git-svn-id: svn://svn.chromium.org/blink/trunk@157314 bbb929c8-8fbe-4397-9dbb-9b2b20218538