On domains with plugins (e.g. chromium-review), viewing a diff in a
change before actually viewing the change and then navigating to the
change-view with the 'a' keyboard shortcut can cause the reply dialog
to be improperly placed on the page.
With this change, `_openReplyDialog` centers the dialog after it's
opened.
Bug: Issue 5506
Change-Id: Id72d428be8e6d8492a2c6d90be372163fad7b82d