cc: Move SyncedScrollOffset to scroll tree

cc/proto/synced_property.proto

Index: cc/proto/synced_property.proto
diff --git a/cc/proto/synced_property.proto b/cc/proto/synced_property.proto
new file mode 100644
index 0000000000000000000000000000000000000000..5b8658f341125055a0bf0e2a8fd7cd4ad2e68f4e
--- /dev/null
+++ b/cc/proto/synced_property.proto
@@ -0,0 +1,29 @@
+// Copyright 2016 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 "scroll_offset.proto";
+
+option optimize_for = LITE_RUNTIME;
+
+package cc.proto;
+
+// Since plumbing SyncedScrollOffset is only used by PropertyTree, and can only
+// travel from main thread to impl thread one way, the From/To protobuf function
+// for SyncedProperty only needs to care the current base, because main thread
+// does not have pending/active tree or scroll offset deltas.
+message ScrollOffsetGroup {
+  optional ScrollOffset pending_base = 1;
+  optional ScrollOffset pending_delta = 2;
+  optional ScrollOffset active_base = 3;
+  optional ScrollOffset active_delta = 4;
+  optional ScrollOffset sent_delta = 5;
+}
+
+message SyncedProperty {
+  optional bool clobber_active_value = 1;
+
+  optional ScrollOffsetGroup scroll_offset_group = 1001;
+}