Make TypingCommand::insertText() to take SelectionInDOMTree instead of VisibleSelection

This patch makes |TypingCommand::insertText()| to take |SelectionInDOMTree|
instead of |VisibleSelection| to reduce usage of |VisibleSelection| for
improving code health.

BUG=657237
TEST=n/a

Review-Url: https://codereview.chromium.org/2733183002
Cr-Commit-Position: refs/heads/master@{#455368}
TypingCommand* Editor::lastTypingCommandIfStillOpenForTyping() const {
  return TypingCommand::lastTypingCommandIfStillOpenForTyping(&frame());
}

TypingCommand* Editor::lastTypingCommandIfStillOpenForTyping() const {
  return TypingCommand::lastTypingCommandIfStillOpenForTyping(&frame());
}
