Setup LayerTree class, refactor 2 functions from LayerTreeHost to it.

cc/proto/layer_tree.proto

Index: cc/proto/layer_tree.proto
diff --git a/cc/proto/region.proto b/cc/proto/layer_tree.proto
similarity index 66%
copy from cc/proto/region.proto
copy to cc/proto/layer_tree.proto
index a4d235be3c5f6e0219910580f11a13a12fc4a503..790f08e672fef6063cd0714bab039d244287a0db 100644
--- a/cc/proto/region.proto
+++ b/cc/proto/layer_tree.proto
@@ -4,12 +4,11 @@
 
 syntax = "proto2";
 
-import "rect.proto";
+package cc.proto;
 
 option optimize_for = LITE_RUNTIME;
 
-package cc.proto;
-
-message Region {
-  repeated Rect rects = 1;
+message LayerTree {
+  repeated int32 layers_that_should_push_properties = 1;

[vmpstr, 2016/07/28 21:43:22]

Maybe we should name these with "id" in the name, kind of like
layer_ids_that_should_push_properties?

+  optional bool in_paint_layer_contents = 2;

[vmpstr, 2016/07/28 21:43:22]

This looks like it's a transitive state that shouldn't be serialized. That is,
it is temporarily true, but mostly false. 

[Khushal, 2016/07/28 21:52:20]

Yup, the aim is to move stuff that actually needs to be serialized into seperate
structs, so both computed and helper variables are not serialized.

 }