Revert "Disable failing background-attachment-local-scrolling.htm"

This reverts commit edf0c2bf.

Reason for revert: The CL that caused the failure has been reverted. 

Original change's description:
> Disable failing background-attachment-local-scrolling.htm
>
> TBR=pdr@chromium.org,wangxianzhu@chromium.org
>
> Bug: 1166540,1046544
> Change-Id: I37ae96bb63da98c90fd85875742b5c76abf34e7b
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628948
> Reviewed-by: Andy Paicu <andypaicu@chromium.org>
> Commit-Queue: Andy Paicu <andypaicu@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#843444}

TBR=andypaicu@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: Iad302719be50e7aaf3915a0816009cd0cf43fba3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1166540
Bug: 1046544
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628384Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843602}

third_party/blink/web_tests/TestExpectations

@@ -5847,8 +5847,5 @@ crbug.com/1163793 http/tests/inspector-protocol/page/page-events-associated.js [
 # Sheriff 2021-01-13
 crbug.com/1164459 [ Mac10.15 ] external/wpt/preload/download-resources.html [ Pass Failure ]
 
-# Sheriff 2021-01-14
-crbug.com/1166540 [ Mac ] ietestcenter/css3/bordersbackgrounds/background-attachment-local-scrolling.htm [ Pass Failure ]
-
 # CSS spelling and grammar features (issue 1163437)
 crbug.com/1147859 external/wpt/css/css-pseudo/highlight-painting-order.html [ Failure ]