cc: Implement RemoveLast for DisplayItemList.

cc/base/list_container.cc

Index: cc/base/list_container.cc
diff --git a/cc/base/list_container.cc b/cc/base/list_container.cc
index df21a5b2e486a144736e9c817c66aa30d5acccda..e91a45bf84b236abf104fb70f9141f182e9ce8c4 100644
--- a/cc/base/list_container.cc
+++ b/cc/base/list_container.cc
@@ -53,6 +53,7 @@ class ListContainerBase::ListContainerCharAllocator {
       --capacity;
     }
 
+    bool IsEmpty() const { return !size; }
     bool IsFull() { return capacity == size; }
     size_t NumElementsAvailable() const { return capacity - size; }
 
@@ -62,6 +63,11 @@ class ListContainerBase::ListContainerCharAllocator {
       return LastElement();
     }
 
+    void RemoveLast() {
+      DCHECK(!IsEmpty());
+      --size;
+    }
+
     char* Begin() const { return data.get(); }
     char* End() const { return data.get() + size * step; }
     char* LastElement() const { return data.get() + (size - 1) * step; }
@@ -121,6 +127,14 @@ class ListContainerBase::ListContainerCharAllocator {
     AllocateNewList(initial_allocation_size);
   }
 
+  void RemoveLast() {
+    DCHECK(!IsEmpty());
+    last_list_->RemoveLast();
+    if (last_list_->IsEmpty())
+      DeallocateLastList();

[danakj, 2015/06/02 20:04:32]

Is it possible to avoid this deallocate and just leave the empty last list
around with size 0?

This could lead to a huge performance cliff, and it'll be rare/random/hard to
reproduce.

+    --size_;
+  }
+
   void Erase(PositionInListContainerCharAllocator position) {
     DCHECK_EQ(this, position.ptr_to_container);
     storage_[position.vector_index]->Erase(position.item_iterator);
@@ -170,6 +184,15 @@ class ListContainerBase::ListContainerCharAllocator {
   }
 
  private:
+  void DeallocateLastList() {
+    // Don't deallocate the only list; we always expect there to be one.
+    if (list_count_ <= 1)
+      return;
+    --list_count_;
+    storage_.pop_back();
+    last_list_ = list_count_ ? storage_.back() : nullptr;
+  }
+
   ScopedPtrVector<InnerList> storage_;
   const size_t element_size_;
   size_t size_;
@@ -275,6 +298,10 @@ ListContainerBase::ListContainerBase(size_t max_size_for_derived_class,
 ListContainerBase::~ListContainerBase() {
 }
 
+void ListContainerBase::RemoveLast() {
+  data_->RemoveLast();
+}
+
 void ListContainerBase::EraseAndInvalidateAllPointers(
     ListContainerBase::Iterator position) {
   data_->Erase(position);