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}
static void dispatchEditableContentChangedEvents(Element* startRoot,
                                                 Element* endRoot) {
  if (startRoot)
    startRoot->dispatchEvent(
        Event::create(EventTypeNames::webkitEditableContentChanged));
  if (endRoot && endRoot != startRoot)
    endRoot->dispatchEvent(
        Event::create(EventTypeNames::webkitEditableContentChanged));
}

static void dispatchEditableContentChangedEvents(Element* startRoot,
                                                 Element* endRoot) {
  if (startRoot)
    startRoot->dispatchEvent(
        Event::create(EventTypeNames::webkitEditableContentChanged));
  if (endRoot && endRoot != startRoot)
    endRoot->dispatchEvent(
        Event::create(EventTypeNames::webkitEditableContentChanged));
}
