Add ScrollTree builder and unit test

cc/proto/layer.proto

Index: cc/proto/layer.proto
diff --git a/cc/proto/layer.proto b/cc/proto/layer.proto
index 4c8ba3197a9c4dbf7b670fd453a2fe3957659f87..50b6fa31d47b1d5ce12dfbcd2a59df7413cf5b19 100644
--- a/cc/proto/layer.proto
+++ b/cc/proto/layer.proto
@@ -76,59 +76,60 @@ message BaseLayerProperties {
   optional int64 transform_free_index = 4;
   optional int64 effect_tree_index = 5;
   optional int64 clip_tree_index = 6;
-  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;
+  optional int64 scroll_tree_index = 7;
+  optional Vector2dF offset_to_transform_parent = 8;
+  optional bool double_sided = 9;
+  optional bool draws_content = 10;
+  optional bool hide_layer_and_subtree = 11;
+  optional bool has_render_surface = 12;
   // 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;
-  optional bool have_wheel_event_handlers = 16;
-  optional bool have_scroll_event_handlers = 17;
-  optional Region non_fast_scrollable_region = 18;
-  optional Region touch_event_handler_region = 19;
-  optional int32 scroll_blocks_on = 20;
-  optional bool contents_opaque = 21;
-  optional float opacity = 22;
-  optional SkXfermode.Mode blend_mode = 23;
-  optional bool is_root_for_isolated_group = 24;
-  optional PointF position = 25;
-  optional bool is_container_for_fixed_position_layers = 26;
-  optional LayerPositionConstraint position_constraint = 27;
-  optional bool should_flatten_transform = 28;
-  optional bool should_flatten_transform_from_property_tree = 29;
-  optional int32 num_layer_or_descendants_with_copy_request = 30;
-  optional SkXfermode.Mode draw_blend_mode = 31;
-  optional bool use_parent_backface_visibility = 32;
-  optional Transform transform = 33;
-  optional bool transform_is_invertible = 34;
-  optional int32 sorting_context_id = 35;
-  optional int32 num_descendants_that_draw_content = 36;
-
-  optional int32 scroll_clip_layer_id = 37;
-  optional bool user_scrollable_horizontal = 38;
-  optional bool user_scrollable_vertical = 39;
-
-  optional int32 scroll_parent_id = 40;
-  repeated int32 scroll_children_ids = 41;
-
-  optional int32 clip_parent_id = 42;
-  repeated int32 clip_children_ids = 43;
-
-  optional ScrollOffset scroll_offset = 44;
-  optional Vector2dF scroll_compensation_adjustment = 45;
-
-  optional Rect update_rect = 46;
-  optional bool stacking_order_changed = 47;
+  // repeated FilterOperation filters = 13;
+  // repeated FilterOperation background_filters = 14;
+  optional bool masks_to_bounds = 15;
+  optional uint32 main_thread_scrolling_reasons = 16;
+  optional bool have_wheel_event_handlers = 17;
+  optional bool have_scroll_event_handlers = 18;
+  optional Region non_fast_scrollable_region = 19;
+  optional Region touch_event_handler_region = 20;
+  optional int32 scroll_blocks_on = 21;
+  optional bool contents_opaque = 22;
+  optional float opacity = 23;
+  optional SkXfermode.Mode blend_mode = 24;
+  optional bool is_root_for_isolated_group = 25;
+  optional PointF position = 26;
+  optional bool is_container_for_fixed_position_layers = 27;
+  optional LayerPositionConstraint position_constraint = 28;
+  optional bool should_flatten_transform = 29;
+  optional bool should_flatten_transform_from_property_tree = 30;
+  optional int32 num_layer_or_descendants_with_copy_request = 31;
+  optional SkXfermode.Mode draw_blend_mode = 32;
+  optional bool use_parent_backface_visibility = 33;
+  optional Transform transform = 34;
+  optional bool transform_is_invertible = 35;
+  optional int32 sorting_context_id = 36;
+  optional int32 num_descendants_that_draw_content = 37;
+
+  optional int32 scroll_clip_layer_id = 38;
+  optional bool user_scrollable_horizontal = 39;
+  optional bool user_scrollable_vertical = 40;
+
+  optional int32 scroll_parent_id = 41;
+  repeated int32 scroll_children_ids = 42;
+
+  optional int32 clip_parent_id = 43;
+  repeated int32 clip_children_ids = 44;
+
+  optional ScrollOffset scroll_offset = 45;
+  optional Vector2dF scroll_compensation_adjustment = 46;
+
+  optional Rect update_rect = 47;
+  optional bool stacking_order_changed = 48;
 
   // TODO(nyquist): Figure out what to do with LayerAnimationController.
-  // optional LayerAnimationController layer_animation_controller = 48;
+  // optional LayerAnimationController layer_animation_controller = 49;
 
   // TODO(nyquist): Figure out what to do with FrameTimingRequests.
-  // repeated FrameTimingRequest frame_timing_requests = 49;
+  // repeated FrameTimingRequest frame_timing_requests = 50;
 }
 
 message PictureLayerProperties {