[@scope] Pass TreeScope separately to SelectorChecker

The `scope` member of SelectorCheckingContext is currently used
for several closely related (but different) things, causing subtle
bugs and a hard-to-understand model.

To address this, pass the TreeScope to SelectorChecker separately,
and use context.tree_scope for ShadowRoot-dependent decisions
in selector matching.

The `context.scope` field is now only used for two things:

 1. For matching the :scope pseudo-class.
 2. For creating the default activation for @scope.

This fixes Issue 377647716, because since we no longer "clobber"
context.scope during scope activation, making ParentElement
traverse to the shadow host.

Fixed: 377647716
Change-Id: I833f1dfb03c092927712bc6fd649a414a45ac7a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6035173
Reviewed-by: Rune Lillesveen <[email protected]>
Commit-Queue: Anders Hartvoll Ruud <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1386249}

css/css-cascade/scope-shadow.html

@@ -159,3 +159,75 @@
     assert_equals(getComputedStyle(a).zIndex, '2');
   }, 'Implicit @scope in construted stylesheet');
 </script>
+
+<div id=scope_via_parent_pseudo_subject>
+  <div class=host>
+    <template shadowrootmode=open>
+      <style>
+          @scope (:host) {
+            :scope {
+              & {
+                z-index: 1;
+              }
+            }
+          }
+      </style>
+    </template>
+  </div>
+</div>
+<script>
+  test(() => {
+    let host = scope_via_parent_pseudo_subject.querySelector('.host');
+    assert_equals(getComputedStyle(host).zIndex, '1');
+  }, 'Matching :host via &, :scope (subject)');
+</script>
+
+<div id=scope_via_parent_pseudo_non_subject>
+  <div class=host>
+    <template shadowrootmode=open>
+      <style>
+          @scope (:host) {
+            :scope .a {
+              & {
+                z-index: 1;
+              }
+            }
+          }
+      </style>
+      <div class=a>
+      </div>
+    </template>
+  </div>
+</div>
+<script>
+  test(() => {
+    let host = scope_via_parent_pseudo_non_subject.querySelector('.host');
+    let a = host.shadowRoot.querySelector('.a');
+    assert_equals(getComputedStyle(a).zIndex, '1');
+  }, 'Matching :host via &, :scope (non-subject)');
+</script>
+
+<div id=scope_via_parent_pseudo_non_subject_child>
+  <div class=host>
+    <template shadowrootmode=open>
+      <style>
+          @scope (:host) {
+            :scope > .a {
+              & {
+                z-index: 1;
+              }
+            }
+          }
+      </style>
+      <div class=a>
+      </div>
+    </template>
+  </div>
+</div>
+<script>
+  test(() => {
+    let host = scope_via_parent_pseudo_non_subject_child.querySelector('.host');
+    let a = host.shadowRoot.querySelector('.a');
+    assert_equals(getComputedStyle(a).zIndex, '1');
+  }, 'Matching :host via &, :scope (non-subject, >)');
+</script>