Keep br while replacing plain texts with content ending with newline. 

The <br> after the insert position was removed while inserting a text ending with a newline. This caused the loss of the newline at the end of the inserted text. This fix adds extra condition checking for this scenario and keeps the <br> while replacing plain texts with content ending with newline.


BUG=121627

Review URL: https://codereview.chromium.org/113663003

git-svn-id: svn://svn.chromium.org/blink/trunk@165565 bbb929c8-8fbe-4397-9dbb-9b2b20218538