Add ScrollTree builder and unit test

cc/proto/layer.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 "display_list_recording_source.proto";
 import "layer_position_constraint.proto";
 import "point3f.proto";
 import "pointf.proto";
@@ skipping 58 matching lines @@
   optional PictureLayerProperties picture = 6;
 }
 
 message BaseLayerProperties {
   optional Point3F transform_origin = 1;
   optional uint32 background_color = 2;
   optional Size bounds = 3;
   optional int64 transform_free_index = 4;
   optional int64 effect_tree_index = 5;
   optional int64 clip_tree_index = 6;
+  // NEXT ID: 51

[David Trainor- moved to gerrit, 2016/01/26 16:34:56]

Ah sorry!  Can you put this up top?

[sunxd, 2016/01/26 16:39:43]

My bad :), move it to the top now. Thanks for pointing out!

+  optional int64 scroll_tree_index = 50;
   optional Vector2dF offset_to_transform_parent = 7;
   optional bool double_sided = 8;
   optional bool draws_content = 9;
   optional bool hide_layer_and_subtree = 10;
   optional bool has_render_surface = 11;
   // TODO(nyquist): Add support for FilterOperation. See crbug.com/541321.
   // repeated FilterOperation filters = 12;
   // repeated FilterOperation background_filters = 13;
   optional bool masks_to_bounds = 14;
   optional uint32 main_thread_scrolling_reasons = 15;
@@ skipping 44 matching lines @@
 
 message PictureLayerProperties {
   optional DisplayListRecordingSource recording_source = 1;
   optional Region invalidation = 2;
   optional Rect last_updated_visible_layer_rect = 3;
   optional bool is_mask = 4;
   optional bool nearest_neighbor = 5;
 
   optional int64 update_source_frame_number = 6;
 }