Loosen containment rules on dummy scrollbar elements

This commit uses `content` containment (i.e. `layout paint style`) as
opposed to `strict` containment (i.e. `layout paint style size`) for
dummy scrollbar elements.

By removing `size` containment we are fixing a rendering bug that was
preventing the scrollbar from being sized correctly. This problem was
caught by a TextEditorComponent test
(https://circleci.com/gh/atom/atom/6393).

src/text-editor-component.js

@@ -3023,7 +3023,7 @@ class DummyScrollbarComponent {
 
     const outerStyle = {
       position: 'absolute',
-      contain: 'strict',
+      contain: 'content',
       zIndex: 1,
       willChange: 'transform'
     }