[cascade] Use StyleCascade for StyleForPage

Also add DCHECK to verify that nothing uses StyleResolver::
ApplyMatchedProperties when CSSCascade is enabled.

Bug: 947004
Change-Id: I201b2022050ac9df3a59edf7119c4cf3f46db67a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2041619
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740040}

third_party/blink/renderer/core/css/resolver/style_resolver.cc

@@ -1113,16 +1113,23 @@ scoped_refptr<const ComputedStyle> StyleResolver::StyleForPage(int page_index) {
 
   NeedsApplyPass needs_apply_pass;
   const MatchResult& result = collector.MatchedResult();
-  ApplyMatchedProperties<kAnimationPropertyPriority, kUpdateNeedsApplyPass>(
-      state, result.AllRules(), false, inherited_only, needs_apply_pass);
-  ApplyMatchedProperties<kHighPropertyPriority, kCheckNeedsApplyPass>(
-      state, result.AllRules(), false, inherited_only, needs_apply_pass);
 
-  // If our font got dirtied, go ahead and update it now.
-  UpdateFont(state);
+  if (RuntimeEnabledFeatures::CSSCascadeEnabled()) {
+    StyleCascade cascade(state);
+    cascade.Analyze(result, CascadeFilter());
+    cascade.Apply(result, CascadeFilter());
+  } else {
+    ApplyMatchedProperties<kAnimationPropertyPriority, kUpdateNeedsApplyPass>(
+        state, result.AllRules(), false, inherited_only, needs_apply_pass);
+    ApplyMatchedProperties<kHighPropertyPriority, kCheckNeedsApplyPass>(
+        state, result.AllRules(), false, inherited_only, needs_apply_pass);
 
-  ApplyMatchedProperties<kLowPropertyPriority, kCheckNeedsApplyPass>(
-      state, result.AllRules(), false, inherited_only, needs_apply_pass);
+    // If our font got dirtied, go ahead and update it now.
+    UpdateFont(state);
+
+    ApplyMatchedProperties<kLowPropertyPriority, kCheckNeedsApplyPass>(
+        state, result.AllRules(), false, inherited_only, needs_apply_pass);
+  }
 
   LoadPendingResources(state);
 
@@ -1557,6 +1564,8 @@ void StyleResolver::ApplyMatchedProperties(StyleResolverState& state,
                                            bool inherited_only,
                                            NeedsApplyPass& needs_apply_pass,
                                            ForcedColorFilter forced_colors) {
+  DCHECK(!RuntimeEnabledFeatures::CSSCascadeEnabled());
+
   if (range.IsEmpty())
     return;