cc: Move SyncedScrollOffset to scroll tree

cc/proto/synced_property.proto

+// 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;
+}