Support Style Sharing for Shadow DOM V1

third_party/WebKit/Source/core/css/resolver/SharedStyleFinder.cpp

Index: third_party/WebKit/Source/core/css/resolver/SharedStyleFinder.cpp
diff --git a/third_party/WebKit/Source/core/css/resolver/SharedStyleFinder.cpp b/third_party/WebKit/Source/core/css/resolver/SharedStyleFinder.cpp
index 842676fb717292ef5d9e3a5e5a3593b4f4c2d903..7cf33826a864486c11d433f0b03da1c9ea904fa3 100644
--- a/third_party/WebKit/Source/core/css/resolver/SharedStyleFinder.cpp
+++ b/third_party/WebKit/Source/core/css/resolver/SharedStyleFinder.cpp
@@ -198,6 +198,13 @@ bool SharedStyleFinder::sharingCandidateCanShareHostStyles(
   return elementShadow->hasSameStyles(*candidateShadow);
 }
 
+bool SharedStyleFinder::sharingCandidateAssignedToSameSlot(
+    Element& candidate) const {
+  HTMLSlotElement* slot1 = element().assignedSlot();
+  HTMLSlotElement* slot2 = candidate.assignedSlot();
+  return slot1 && slot2 && slot1 == slot2;

[rune, 2017/01/11 11:34:27]

The most common case is that slot1 and slot2 are both null. Won't this skip
style sharing for the common case?

+}
+
 bool SharedStyleFinder::sharingCandidateDistributedToSameInsertionPoint(
     Element& candidate) const {
   HeapVector<Member<InsertionPoint>, 8> insertionPoints,
@@ -251,6 +258,10 @@ bool SharedStyleFinder::canShareStyleWithElement(Element& candidate) const {
     return false;
   if (!sharingCandidateCanShareHostStyles(candidate))
     return false;
+  // For Shadow DOM V1
+  if (!sharingCandidateAssignedToSameSlot(candidate))
+    return false;
+  // For Shadow DOM V0
   if (!sharingCandidateDistributedToSameInsertionPoint(candidate))
     return false;
   if (candidate.isInTopLayer() != element().isInTopLayer())