Reset virtual time state in scrollbar tests

This prevents ScrollbarTestWithVirtualTimer from polluting global state
for tests following it.

Bug: 791742
Change-Id: Iae3440451833408a6a5bd24b3319b307cd6d3547
Reviewed-on: https://chromium-review.googlesource.com/969582
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544691}

third_party/WebKit/Source/core/layout/ScrollbarsTest.cpp

@@ -100,6 +100,11 @@ class ScrollbarsTestWithVirtualTimer : public ScrollbarsTest {
     WebView().Scheduler()->EnableVirtualTime();
   }
 
+  void TearDown() override {
+    WebView().Scheduler()->DisableVirtualTimeForTesting();
+    ScrollbarsTest::TearDown();
+  }
+
   void TimeAdvance() {
     WebView().Scheduler()->SetVirtualTimePolicy(
         PageScheduler::VirtualTimePolicy::kAdvance);