[blimp] Add SkPicture caching support.

cc/proto/layer_tree_host.proto

Index: cc/proto/layer_tree_host.proto
diff --git a/cc/proto/layer_tree_host.proto b/cc/proto/layer_tree_host.proto
index 50616332076460598b8e6f60a2e03cfd887424ea..4d89190f9d496780fdf13c77ece80658e8e87934 100644
--- a/cc/proto/layer_tree_host.proto
+++ b/cc/proto/layer_tree_host.proto
@@ -4,6 +4,7 @@
 
 syntax = "proto2";
 
+import "display_item.proto";
 import "layer.proto";
 import "layer_selection_bound.proto";
 import "layer_tree_debug_state.proto";
@@ -15,6 +16,15 @@ package cc.proto;
 
 option optimize_for = LITE_RUNTIME;
 
+message SkPictureData {
+  optional SkPictureID id = 1;
+  optional bytes payload = 2; /* SkData */
+}
+
+message SkPictures {
+  repeated SkPictureData pictures = 1;
+}
+
 message LayerTreeHost {
   // Not all members of LayerTreeHost are serialized, as they are not helpful
   // for remote usage. See implementation of
@@ -57,4 +67,5 @@ message LayerTreeHost {
   optional uint32 touch_start_or_move_event_listener_properties = 36;
   repeated int32 layers_that_should_push_properties = 37;
   optional uint32 touch_end_or_cancel_event_listener_properties = 38;
+  optional SkPictures pictures = 39;
 }