[content-visibility]: Ensure to ignore hidden-matchable children.

This patch ensures that we property check that hidden-matchable
children should be skipped. This only affects hidden-matchable, because
hidden hides everything to the buggy if statement would work and
auto allows all activations so the result of the buggy return is
the same as it would have been without the bug.

The effect of this bug is that hidden-matchable children are exposed
to accessibility, which they shouldn't be.

R=chrishtr@chromium.org, jarhar@chromium.org

Change-Id: I863889889cb28e4d3ab075f599f7660dc8637cff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2567342Reviewed-by: Joey Arhar <jarhar@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832495}

third_party/blink/renderer/core/display_lock/display_lock_utilities.cc

@@ -398,9 +398,6 @@ bool DisplayLockUtilities::IsInUnlockedOrActivatableSubtree(
       node.GetDocument()
               .GetDisplayLockDocumentState()
               .LockedDisplayLockCount() == 0 ||
-      node.GetDocument()
-              .GetDisplayLockDocumentState()
-              .DisplayLockBlockingAllActivationCount() == 0 ||
       !node.CanParticipateInFlatTree()) {
     return true;
   }

third_party/blink/renderer/core/display_lock/display_lock_utilities_test.cc

@@ -37,6 +37,21 @@ class DisplayLockUtilitiesTest
   }
 };
 
+TEST_F(DisplayLockUtilitiesTest, ShouldIgnoreHiddenMatchableChildren) {
+  SetBodyInnerHTML(R"HTML(
+    <style>
+    .hidden { content-visibility: hidden-matchable }
+    </style>
+    <div class=hidden>
+      <div id=target></div>
+    </div>
+  )HTML");
+
+  Node* target = GetDocument().getElementById("target");
+  EXPECT_TRUE(DisplayLockUtilities::ShouldIgnoreNodeDueToDisplayLock(
+      *target, DisplayLockActivationReason::kAccessibility));
+}
+
 TEST_F(DisplayLockUtilitiesTest, DISABLED_ActivatableLockedInclusiveAncestors) {
   SetBodyInnerHTML(R"HTML(
     <style>