Added serialization to protobufs for property trees.

cc/proto/scroll_offset.proto

Index: cc/proto/scroll_offset.proto
diff --git a/cc/proto/pointf.proto b/cc/proto/scroll_offset.proto
similarity index 76%
copy from cc/proto/pointf.proto
copy to cc/proto/scroll_offset.proto
index e0aac584f274dd1c27cc293e1b91879beecb0267..5b9b74b465061b2f87cd559a2b35a51fc3167bdb 100644
--- a/cc/proto/pointf.proto
+++ b/cc/proto/scroll_offset.proto
@@ -8,7 +8,7 @@ option optimize_for = LITE_RUNTIME;
 
 package cc.proto;
 
-message PointF {
-  optional float x = 1;
-  optional float y = 2;
-}
+message ScrollOffset {
+  optional double x = 1;
+  optional double y = 2;
+}