[css-contain] Link failures in WPT tests to the related bugs

Two of the failures are related to stacking contexts:
* external/wpt/css/css-contain/contain-paint-021.html
* external/wpt/css/css-contain/contain-layout-016.html

The other one was a mistake that has been already fixed in WPT
(see https://github.com/web-platform-tests/wpt/pull/12314):
* external/wpt/css/css-contain/contain-paint-001.html

TBR=eae@chromium.org

BUG=870811,868102,870157

Change-Id: I60e89decd2f939d1d9a850a1b8b556d31658e872
Reviewed-on: https://chromium-review.googlesource.com/1163673Reviewed-by: Manuel Rego <rego@igalia.com>
Commit-Queue: Manuel Rego <rego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#580878}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2702,9 +2702,9 @@ crbug.com/849859 external/wpt/web-animations/timing-model/animations/pausing-an-
 crbug.com/870526 external/wpt/payment-handler/idlharness.https.any.serviceworker.html [ Skip ]
 
 # ====== New tests from wpt-importer added here ======
-crbug.com/626703 external/wpt/css/css-contain/contain-paint-021.html [ Failure ]
-crbug.com/626703 external/wpt/css/css-contain/contain-paint-001.html [ Failure ]
-crbug.com/626703 external/wpt/css/css-contain/contain-layout-016.html [ Failure ]
+crbug.com/868102 external/wpt/css/css-contain/contain-paint-021.html [ Failure ]
+crbug.com/870811 external/wpt/css/css-contain/contain-paint-001.html [ Failure ]
+crbug.com/870157 external/wpt/css/css-contain/contain-layout-016.html [ Failure ]
 crbug.com/626703 external/wpt/clear-site-data/executionContexts.sub.html [ Timeout ]
 crbug.com/626703 [ Win10 ] external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-column-row-gap-004.html [ Failure ]
 crbug.com/626703 [ Linux Mac Win7 ] external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-column-row-gap-004.html [ Failure ]