More accurate scrollable state

Use more accurate scrollability check that matches the scrollable state
with the presence of a scrollbar. Use the same technique to determine
scrollability as blink/renderer/core/page/spatial_navigation.cc

Also improves the AccessibilityScrollableOverflow dump tree test:
- Class name matches scrollable state for easy debugging
- Loading test page reveals green dashed line around expected
  scrollable elems, red solid line around any elements expected
  to not be scrollable.
- Test more cases of odd x-hidden and y-hidden cases. In the x-hidden
  case, the renderer makes this content vertically scrollable if there
  content.

Bug: None
Change-Id: I10f266e65ad5442ad20caf25a7b94c9dfd2192ef
Cq-Do-Not-Cancel-Tryjobs: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2120105
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754153}