GitLab

* Add sliders to role conversions
* Add increment and decrement actions
* Update data structures in FakeSemanticTree
* Add waits for node updates in FakeSemanticTree
* Add test only action/event pair in browsertests so waiting for actions
  is more reliable

Test: AccessibilityBridgeTest.Slider
Bug: fuchsia:56295, 1122806
Change-Id: Id0658ada5b15da1c128b69c1346aee1d39f2a6f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314758
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812919}