Bug 959201 - [Messages][Drafts] Wrong Cursor position in the message compose of a Draft

Put the cursor at the end of the message when opening a draft.
Please note we could have some conditions that make the number of message child nodes more than 2:
1) Text message with new line break
2) MMS with attachment

In these cases, you could see the cursor is still not in the correct position. So please set ```dom.message.childNodes.length - 1``` as the offset parameter to make sure the cursor always on the last element