Enabled scrollbar-button-gesture-target.html

Root cause for the test failure was fixed in:
https://crrev.com/c/1865529

Change-Id: I513c8948803be3e432a4331920e5454b717f9ad7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2059048Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#742292}

third_party/blink/web_tests/TestExpectations

@@ -5984,10 +5984,6 @@ crbug.com/937811 [ Win Release ] http/tests/devtools/elements/shadow/elements-pa
 crbug.com/705583 external/wpt/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_navigate_history_go_back.html [ Skip ]
 crbug.com/705583 external/wpt/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_navigate_history_go_forward.html [ Skip ]
 
-# Sheriff 2019-07-30
-crbug.com/987115 [ Win ] virtual/threaded-prefer-compositing/fast/scrolling/scrollbars/scrollbar-button-gesture-target.html [ Crash Pass Failure ]
-crbug.com/987115 [ Linux ] virtual/threaded-prefer-compositing/fast/scrolling/scrollbars/scrollbar-button-gesture-target.html [ Crash Pass Failure ]
-
 # Sheriff 2019-07-31
 crbug.com/979422 [ Mac ] fast/borders/border-radius-mask-canvas-with-shadow.html [ Failure ]
 crbug.com/979422 [ Mac ] fast/borders/border-radius-mask-canvas-all.html [ Failure ]