Reenable scroll-animations/element-based-offset tests

The underlying source of the test flakes was likely addressed in
recent changes to the handling of play and pause for scroll-linked
animations.

https://chromium-review.googlesource.com/c/chromium/src/+/2386339

This patch also contains a cleanup to remove the unnecessarily
convoluted way of hiding the scrollbar for the clamped offset test.

Tested with 1000+ retries of the two tests in a virtual threaded
environment.

Bug: 1080609
Change-Id: If9097b0a2017a9c87a359b61ba93f35bae56b512
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2475935Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817568}

third_party/blink/web_tests/TestExpectations

@@ -145,7 +145,7 @@ virtual/composite-after-paint/paint/frames/* [ Pass ]
 virtual/composite-after-paint/scrollingcoordinator/* [ Pass ]
 # --- End CompositeAfterPaint Tests --
 
-# --- BEGIN OOP-R Canvas tests --- 
+# --- BEGIN OOP-R Canvas tests ---
 crbug.com/1081534 [ Win7 ] virtual/oopr-canvas2d/fast/canvas/canvas-composite-video-shadow.html [ Pass Failure ]
 crbug.com/1081534 [ Win ] virtual/oopr-canvas2d/fast/canvas/canvas-clearRect-first.html [ Failure ]
 crbug.com/1081534 [ Win ] virtual/oopr-canvas2d/fast/canvas/canvas-clearRect-with-clip.html [ Failure ]
@@ -6194,9 +6194,6 @@ crbug.com/1011811 http/tests/devtools/network/network-xhr-data-received-async-re
 crbug.com/1011811 http/tests/devtools/network/download.js [ Pass Failure ]
 crbug.com/1011811 http/tests/devtools/sxg/sxg-disable-cache.js [ Pass Failure ]
 
-crbug.com/1080609 virtual/threaded/external/wpt/scroll-animations/element-based-offset.html [ Pass Failure ]
-crbug.com/1080609 virtual/threaded/external/wpt/scroll-animations/element-based-offset-clamp.html [ Pass Failure ]
-
 # Fails when test moved to use full compositor pipe.
 crbug.com/1085832 [ Fuchsia ] images/size-failure.html [ Timeout ]
 

third_party/blink/web_tests/external/wpt/scroll-animations/element-based-offset-clamp.html

@@ -7,25 +7,19 @@
 <script src="testcommon.js"></script>
 
 <style>
+/*
+ * Overflow hidden prevents user scroll including mouse wheel; however, the
+ * element is still a scrollable container and can be scrolled programmatically.
+ * Removing the visible scrollbars in this manner simplifies the position
+ * calculations in the text.
+ */
 .scroller {
-  overflow: scroll;
+  overflow: hidden;
   height: 500px;
   width: 500px;
   will-change: transform;
 }
 
-/* Disable scrollbars to simplify the calculations in the test. */
-.scroller {
-  scrollbar-width: 0;
-}
-/*
-Chrome does not support scrollbar-width so we use this non-standard property
-until it does.
-*/
-.scroller::-webkit-scrollbar {
-  display: none;
-}
-
 .contents {
   height: 1200px;
   width: 1200px;
@@ -212,4 +206,4 @@ until it does.
       createScrollAnimationTest(description, config);
     }
   }
-</script>
+</script>
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