Don't allow the composited-alpha folding optimization to cross subsequences

third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp

Index: third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp b/third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp
index 0b017b04664f39301d876849fb16377bc756bc75..aa0b0378feb0fd04b0e561c378f6e5d6b7cb978f 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp
+++ b/third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp
@@ -163,6 +163,7 @@ void PaintController::AddCachedSubsequence(const DisplayItemClient& client,
          new_cached_subsequences_.end());
 
   new_cached_subsequences_.insert(&client, SubsequenceMarkers(start, end));
+  last_cached_subsequence_end_ = end;
 }
 
 bool PaintController::LastDisplayItemIsNoopBegin() const {
@@ -173,6 +174,11 @@ bool PaintController::LastDisplayItemIsNoopBegin() const {
   return last_display_item.IsBegin() && !last_display_item.DrawsContent();
 }
 
+bool PaintController::LastDisplayItemIsSubsequenceEnd() const {
+  return !new_cached_subsequences_.IsEmpty() &&
+         last_cached_subsequence_end_ == new_display_item_list_.size() - 1;
+}
+
 void PaintController::RemoveLastDisplayItem() {
   if (new_display_item_list_.IsEmpty())
     return;
@@ -536,6 +542,7 @@ void PaintController::CommitNewDisplayItems(
 
   new_cached_subsequences_.Swap(current_cached_subsequences_);
   new_cached_subsequences_.Clear();
+  last_cached_subsequence_end_ = 0;
   for (auto& item : current_cached_subsequences_) {
     item.key->SetDisplayItemsCached(current_cache_generation_);
 #if CHECK_DISPLAY_ITEM_CLIENT_ALIVENESS