Added mouseMoveTo to stabilize the test.

This CL adds mouseMoveTo API to stabilize the test on Mac.

Bug: 1058244, 1057355
Change-Id: I0be5ce4ffa508319ed58e37ac3e81f9fa5a88a9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2094816Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#748750}

third_party/blink/web_tests/TestExpectations

@@ -6747,9 +6747,6 @@ crbug.com/1058073 [ Mac ] http/tests/devtools/oopif/oopif-elements-navigate-out.
 crbug.com/1058073 [ Mac ] http/tests/devtools/service-workers/sw-navigate-useragent.js [ Pass Failure ]
 crbug.com/1058137 virtual/threaded/http/tests/devtools/tracing/timeline-paint/timeline-paint.js [ Pass Failure ]
 
-# Sheriff 2020-03-04
-crbug.com/1058244 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling/fast/scrolling/scrollbars/scrollbar-rtl-manipulation.html [ Pass Failure ]
-
 # Ecosystem-Infra Sheriff 2020-03-04
 crbug.com/1058403 external/wpt/webaudio/the-audio-api/the-audioworklet-interface/suspended-context-messageport.https.html [ Failure ]
 

third_party/blink/web_tests/fast/scrolling/scrollbars/scrollbar-rtl-manipulation.html

@@ -79,6 +79,7 @@ window.onload = () => {
     // Testing vertical scroll.
     let x = standardRect.left + TRACK_WIDTH / 2;
     let y = standardRect.top + (onMacPlatform ? 0 : BUTTON_WIDTH) + 2;
+    await mouseMoveTo(x, y);
     await mouseDownAt(x, y);
     await mouseMoveTo(x, y + 15);
     await mouseUpAt(x, y + 15);
@@ -88,6 +89,7 @@ window.onload = () => {
     // Testing horizontal scroll.
     x = standardRect.right - (onMacPlatform ? 0 : BUTTON_WIDTH) - 2;
     y = standardRect.bottom - TRACK_WIDTH / 2;
+    await mouseMoveTo(x, y);
     await mouseDownAt(x, y);
     await mouseMoveTo(x - 15, y);
     await mouseUpAt(x - 15, y);