Because of the way iron-autogrow-textarea works, There will never be
a need to have a vertical scroll bar, as the text area expands
accordingly.
In cases where a horizontal scrollbar appeared (due to long lines of
text), the height of the textarea shrank just enough for a vertical
scrollbar to be added as well. There also appeared to be some browser
quirks in which the scroll bars jumped around when they shouldn't.
In order to prevent this from happening, hide overflow-y.
Additionally, add box-sizing: border-box to all iron-autogrow-textarea
instances, so that scrollbars appear correctly in shadow dom.
Bug: Issue 6500
Change-Id: I28f3b47d656a246decd693f637040748ec4fd3a0
03690330ff