Add fast-path for propagated variable changes (WIP)

third_party/WebKit/Source/core/style/ComputedStyle.cpp

Index: third_party/WebKit/Source/core/style/ComputedStyle.cpp
diff --git a/third_party/WebKit/Source/core/style/ComputedStyle.cpp b/third_party/WebKit/Source/core/style/ComputedStyle.cpp
index 8fc3dab289d1b4d853272aba7112c80f3d00a38b..1fcf1d3bd95be2b0dd94c44703468ad6c44cd534 100644
--- a/third_party/WebKit/Source/core/style/ComputedStyle.cpp
+++ b/third_party/WebKit/Source/core/style/ComputedStyle.cpp
@@ -212,9 +212,14 @@ StyleRecalcChange ComputedStyle::stylePropagationDiff(
 
   bool independentEqual = oldStyle->independentInheritedEqual(*newStyle);
   bool nonIndependentEqual = oldStyle->nonIndependentInheritedEqual(*newStyle);
-  if (!independentEqual || !nonIndependentEqual) {
+  bool variablesEqual = oldStyle->variablesEqual(*newStyle);
+  if (!independentEqual || !nonIndependentEqual || !variablesEqual) {
+    // Check if only independent inherited properties changed.
     if (nonIndependentEqual && !oldStyle->hasExplicitlyInheritedProperties())
-      return IndependentInherit;
+      if (variablesEqual)
+        return IndependentInherit;
+    // Variables and possibly independent inherited properties changed.
+    return IndependentInheritWithVariables;
     return Inherit;
   }
 
@@ -239,6 +244,16 @@ void ComputedStyle::propagateIndependentInheritedProperties(
     setPointerEvents(parentStyle.pointerEvents());
   if (m_nonInheritedData.m_isVisibilityInherited)
     setVisibility(parentStyle.visibility());
+
+  // Variables.
+  if (parentStyle.variables() != variables()) {
+    for (const auto& variable : *parentStyle.variables()) {
+      if (inheritsVariableDefinitionFromParent(variable.key) &&
+          *variable.value != *variables()->getVariable(variable.key)) {
+        setVariable(variable.key, variable.value);
+      }
+    }
+  }
 }
 
 StyleSelfAlignmentData resolvedSelfAlignment(
@@ -518,6 +533,10 @@ bool ComputedStyle::nonIndependentInheritedEqual(
          m_rareInheritedData == other.m_rareInheritedData;
 }
 
+bool ComputedStyle::variablesEqual(const ComputedStyle& other) const {
+  return m_rareInheritedData->compareEqualVariables(*other.m_rareInheritedData);
+}
+
 bool ComputedStyle::loadingCustomFontsEqual(const ComputedStyle& other) const {
   return font().loadingCustomFonts() == other.font().loadingCustomFonts();
 }