Don't insert rednant placeholder for text control after deleting

This patch changes behavior of deleting characters from text control not to insert placeholder when text control is empty or ends with newline after deleting.

Test: editing/deleting/delete-last-char-in-textarea.html
This test covers both 236726 (extra newline) and 236429 (removing needed newline)

This patch also changes tests for new behavior according to 
https://codereview.chromium.org/13532011

BUG=236726

[yosin_UTC9, 2013-05-10 13:49:06]

Could you review this patch?
Thanks in advance.

P.S.
This is my first patch for editing.

[ojan, 2013-05-10 23:38:33]

There was some duplicate effort here unfortunately.
https://codereview.chromium.org/14969020/.

[leviw_travelin_and_unemployed, 2013-05-10 23:47:40]

On 2013/05/10 23:38:33, ojan wrote:
> There was some duplicate effort here unfortunately.
> https://codereview.chromium.org/14969020/.

D'oh... Sorry! Sad to see duplicated effort. I like your change to
HTMLInputElement.cpp, but I like my more generalized solution in
DeleteSelectionCommand.cpp.

[yosin_UTC9, 2013-05-13 04:00:42]

Thanks levi.
https://codereview.chromium.org/14969020/ is better than this patch.

Please include issue 236726 into BUG=.