[not for review - epic CL] Adding Elastic+Momentum+Layered scrolling to views::ScrollView

cc/proto/property_tree.proto

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto2";
 
 import "element_id.proto";
 import "layer_sticky_position_constraint.proto";
 import "rectf.proto";
 import "scroll_offset.proto";
@@ skipping 168 matching lines @@
   // PropertyTree. Only one of these fields should be set, depending on the type
   // of this property tree.
   optional TransformTreeData transform_tree_data = 1000;
   optional ScrollTreeData scroll_tree_data = 1001;
   optional EffectTreeData effect_tree_data = 1002;
 }
 
 message ScrollOffsetMapEntry {
   required int64 layer_id = 1;
   optional SyncedProperty scroll_offset = 2;
+  optional ScrollOffset overscroll = 3;
 }
 
 // Proto for data members of class ScrollTree
 message ScrollTreeData {
   optional int64 currently_scrolling_node_id = 1;
   repeated ScrollOffsetMapEntry layer_id_to_scroll_offset_map = 2;
 }
 
 // Proto for data members of class TransformTree.
 message TransformTreeData {
@@ skipping 29 matching lines @@
   optional int64 sequence_number = 6;
   optional bool is_main_thread = 13;
   optional bool is_active = 14;
   optional bool verify_transform_tree_calculations = 16;
 
   optional Vector2dF inner_viewport_container_bounds_delta = 8;
   optional Vector2dF outer_viewport_container_bounds_delta = 9;
   optional Vector2dF inner_viewport_scroll_bounds_delta = 10;
   repeated int64 always_use_active_tree_opacity_effect_ids = 15 [packed = true];
 }