Fix snap-scrolls-visual-viewport

Add a wait for a scroll event to handle a delay between queuing and
handling of the gesture.

Bug: 878878
Change-Id: I6d887cec2d9206f8a73c8a5cf4eb5a7a8d4baae1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2727600
Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#859117}

third_party/blink/web_tests/TestExpectations

@@ -4856,10 +4856,7 @@ crbug.com/1030258 http/tests/devtools/network/network-cookies-pane.js [ Pass Fai
 crbug.com/1041830 http/tests/devtools/tracing/timeline-js/timeline-js-line-level-profile.js [ Pass Failure ]
 
 # Enable scroll-snap tests on impl thread
-# These are currently failing on Mac which needs more investigation, snap-scrolls-visual-viewport seems flaky
-
 crbug.com/878878 virtual/threaded/fast/scroll-snap/animate-fling-to-snap-points.html [ Failure Pass Timeout ]
-crbug.com/878878 virtual/threaded/fast/scroll-snap/snap-scrolls-visual-viewport.html [ Failure Pass ]
 
 # Sheriff 2019-12-13
 crbug.com/1032451 [ Win7 ] virtual/webrtc-wpt-plan-b/external/wpt/webrtc/idlharness.https.window.html [ Failure Pass ]

third_party/blink/web_tests/fast/scroll-snap/snap-scrolls-visual-viewport.html

@@ -55,16 +55,17 @@
 
   const x = 200;
   const y = 200;
+  const scrollPromise = waitForScrollEvent(document);
   await smoothScroll(scale_factor * scroll_delta,
                      x,
                      y,
                      SOURCE_TYPE,
                      scroll_direction,
                      SPEED_INSTANT);
-
+  await scrollPromise;
   await waitForAnimationEndTimeBased(() => {
-      return window.scrollY + window.visualViewport.offsetTop;
-    });
+    return window.scrollY + window.visualViewport.offsetTop;
+  });
 
   assert_approx_equals(window.scrollY, layout_viewport_y, 1);
   assert_approx_equals(window.visualViewport.offsetTop, visual_viewport_y, 1);