Clean LayerImpl's scroll offset callers in unit tests.

cc/trees/property_tree.h

Index: cc/trees/property_tree.h
diff --git a/cc/trees/property_tree.h b/cc/trees/property_tree.h
index c57ad7591925abe01e9936c8a0c19f832ba6b55b..4e9c9d703edd4d4bf922197208cb42bdee7feff1 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -7,7 +7,7 @@
 
 #include <stddef.h>
 
-#include <unordered_map>
+#include <map>
 #include <vector>
 
 #include "cc/base/cc_export.h"
@@ -566,8 +566,7 @@ class CC_EXPORT ScrollTree final : public PropertyTree<ScrollNode> {
 
   void clear() override;
 
-  typedef std::unordered_map<int, scoped_refptr<SyncedScrollOffset>>
-      ScrollOffsetMap;
+  typedef std::map<int, scoped_refptr<SyncedScrollOffset>> ScrollOffsetMap;
 
   gfx::ScrollOffset MaxScrollOffset(int scroll_node_id) const;
   gfx::Size scroll_clip_layer_bounds(int scroll_node_id) const;
@@ -576,8 +575,8 @@ class CC_EXPORT ScrollTree final : public PropertyTree<ScrollNode> {
   void set_currently_scrolling_node(int scroll_node_id);
   gfx::Transform ScreenSpaceTransform(int scroll_node_id) const;
 
-  // synced_scroll_offset is supposed to be called by Layer/LayerImpl only
   SyncedScrollOffset* synced_scroll_offset(int layer_id);
+  const SyncedScrollOffset* synced_scroll_offset(int layer_id) const;
   void CollectScrollDeltas(ScrollAndScaleSet* scroll_info);
   void UpdateScrollOffsetMap(ScrollOffsetMap* new_scroll_offset_map,
                              LayerTreeImpl* layer_tree_impl);
@@ -585,6 +584,9 @@ class CC_EXPORT ScrollTree final : public PropertyTree<ScrollNode> {
   const ScrollOffsetMap& scroll_offset_map() const;
   void ApplySentScrollDeltasFromAbortedCommit();
   bool SetScrollOffset(int layer_id, const gfx::ScrollOffset& scroll_offset);
+  bool UpdateScrollOffsetBaseForTesting(int layer_id,
+                                        const gfx::ScrollOffset& offset);
+  const gfx::ScrollOffset GetScrollOffsetBaseForTesting(int layer_id) const;
 
  private:
   int currently_scrolling_node_id_;