Auto merge of #15654 - glennw:update-wr-clip-scroll, r=metajack

Update WR (clip scroll group work).

This shouldn't have any noticeable change on Servo, but as we land
various parts of the clip/scroll changes, keeping Servo up to date
will hopefully help catch any regressions early.

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tests/wpt/metadata-css/compositing-1_dev/html/mix-blend-mode-blended-element-overflow-scroll.htm.ini

@@ -1,3 +1,5 @@
 [mix-blend-mode-blended-element-overflow-scroll.htm]
   type: reftest
-  expected: FAIL
+  expected:
+    if os == "linux": FAIL
+

tests/wpt/metadata-css/compositing-1_dev/html/mix-blend-mode-iframe-parent.htm.ini

@@ -1,3 +1,5 @@
 [mix-blend-mode-iframe-parent.htm]
   type: reftest
-  expected: FAIL
+  expected:
+    if os == "linux": FAIL
+

tests/wpt/metadata-css/compositing-1_dev/html/mix-blend-mode-iframe-sibling.htm.ini

@@ -1,3 +1,5 @@
 [mix-blend-mode-iframe-sibling.htm]
   type: reftest
-  expected: FAIL
+  expected:
+    if os == "linux": FAIL
+