Use element id's for animations

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 "rectf.proto";
 import "scroll_offset.proto";
 import "size.proto";
 import "synced_property.proto";
 import "transform.proto";
 import "vector2df.proto";
 
 option optimize_for = LITE_RUNTIME;
 
 package cc.proto;
@@ skipping 88 matching lines @@
   optional int64 num_copy_requests_in_subtree = 8;
   optional int64 transform_id = 9;
   optional int64 clip_id = 10;
   optional int64 target_id = 12;
   optional int64 mask_layer_id = 17;
   optional int64 replica_layer_id = 18;
   optional int64 replica_mask_layer_id = 19;
 }
 
 // Proto for struct ScrollNodeData
-// NEXT ID: 15
+// NEXT ID: 16
 message ScrollNodeData {
   optional bool scrollable = 1;
   optional int32 main_thread_scrolling_reasons = 2;
   optional bool contains_non_fast_scrollable_region = 3;
   optional Size scroll_clip_layer_bounds = 4;
   optional Size bounds = 5;
   optional bool max_scroll_offset_affected_by_page_scale = 6;
   optional bool is_inner_viewport_scroll_layer = 7;
   optional bool is_outer_viewport_scroll_layer = 8;
   optional Vector2dF offset_to_transform_parent = 9;
   optional bool should_flatten = 10;
   optional bool user_scrollable_horizontal = 13;
   optional bool user_scrollable_vertical = 14;
-  optional int64 element_id = 12;
+  optional ElementId element_id = 15;
   optional int64 transform_id = 11;
 }
 
 // This defines the proto used for all types of struct TreeNode.
 message TreeNode {
   // The following fields are the base TreeNode properties. This list
   // corresponds to the data members from struct TreeNode.
   optional int64 id = 1;
   optional int64 parent_id = 2;
   optional int64 owner_id = 3;
@@ skipping 74 matching lines @@
   optional bool full_tree_damaged = 12;
   optional int64 sequence_number = 6;
   optional bool is_main_thread = 13;
   optional bool is_active = 14;
 
   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];
 }