Update scroll-margin WPT to stop flakiness

Updates test 'external/wpt/css/css-scroll-snap/scroll-margin.html' such
that the expected result is consistent. Originally the test's snap
areas were equally distant from the scroll position and therefore the
area chosen was not consistent.

Bug: 964239
Change-Id: I7e8b09719bd526c4e2fb7049131d585ef5e47d8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1799387Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Kaan Alsan <alsan@google.com>
Cr-Commit-Position: refs/heads/master@{#698951}

third_party/blink/web_tests/TestExpectations

@@ -5912,9 +5912,6 @@ crbug.com/963141 [ Mac ] virtual/audio-service/media/video-aspect-ratio.html [ P
 crbug.com/963141 [ Linux ] media/video-object-fit.html [ Pass Failure ]
 crbug.com/963141 [ Linux ] virtual/audio-service/media/video-object-fit.html [ Pass Failure ]
 
-# Sheriff 2019-05-17
-crbug.com/964239 virtual/threaded/external/wpt/css/css-scroll-snap/scroll-margin.html [ Pass Failure ]
-
 # Workaround for bug; should be removed soon.
 crbug.com/959693 external/wpt/trusted-types/WorkerGlobalScope-importScripts.https.html [ Timeout ]
 
@@ -5923,7 +5920,6 @@ crbug.com/972476 std-switch/switch-appearance.html [ Failure ]
 crbug.com/972476 std-switch/switch-appearance-customization.html [ Failure ]
 
 # Sheriff 2019-05-20
-crbug.com/964239 external/wpt/css/css-scroll-snap/scroll-margin.html [ Pass Failure ]
 crbug.com/965389 [ Mac ] media/track/track-cue-rendering-position-auto.html [ Pass Failure ]
 crbug.com/965389 [ Mac ] virtual/audio-service/media/track/track-cue-rendering-position-auto.html [ Pass Failure ]
 

third_party/blink/web_tests/external/wpt/css/css-scroll-snap/scroll-margin.html

@@ -58,14 +58,31 @@ test(() => {
 test(() => {
   target.style.left = "0px";
   target.style.top = "0px";
-
   target.style.scrollSnapAlign = "start";
-  target.style.scrollMargin = "100px";
 
-  // Scroll to the position between #target and #another-target elements but
-  // if the scroll-margin 100px contributed to the snap start-aligned snap
-  // position it will be farther than #another-target.
-  scroller.scrollTo(200, 200);
+  // Since the target is at (0px, 0px) in the scroll port, the added margin
+  // should not be considered, and the snap points for this snap area should be
+  // the closest points in the scroll port (i.e x=0 or y=0).
+  target.style.scrollMargin = "200px";
+
+  // Distance from target without margin:
+  // `scroll position (150px, 150px)` - `target position (0px, 0px)`
+  // = (150px, 150px)
+  //
+  // Distance from target with margin:
+  // `scroll position (150px, 150px)` - [`target position (0px, 0px)` -
+  // `target margin (200px, 200px)`]
+  // = (350px, 350px)
+  //
+  // Distance from other target:
+  // `other target position (400px, 400px)` - `scroll position (150px, 150px)`
+  // = (250px, 250px)
+  //
+  // Therefore if the "out-of-scrollport" scroll-margin contributes to the
+  // calculation, then the other target would be snapped to. However if the
+  // scroll-margin is not considered, then the (0px, 0px) target should be
+  // snapped to.
+  scroller.scrollTo(150, 150);
   assert_equals(scroller.scrollLeft, 0);
   assert_equals(scroller.scrollTop, 0);
 }, "scroll-margin doesn't contribute to the snap position of the element " +

third_party/blink/web_tests/platform/mac/virtual/threaded/external/wpt/css/css-scroll-snap/scroll-margin-expected.txt

-This is a testharness.js-based test.
-PASS Snaps to the positions adjusted by scroll-margin
-FAIL scroll-margin doesn't contribute to the snap position of the element if it's outside of the scroll port assert_equals: expected 0 but got 400
-Harness: the test ran to completion.
-