Clean LayerImpl's scroll offset callers in unit tests.

cc/test/layer_tree_host_common_test.h

 // Copyright 2014 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.
 
 #ifndef CC_TEST_LAYER_TREE_HOST_COMMON_TEST_H_
 #define CC_TEST_LAYER_TREE_HOST_COMMON_TEST_H_
 
 #include <algorithm>
 #include <vector>
 
@@ skipping 141 matching lines @@
   LayerSettings layer_settings_;
 
   int render_surface_layer_list_count_;
 };
 
 class LayerTreeHostCommonTest : public LayerTreeHostCommonTestBase,
                                 public testing::Test {
  public:
   LayerTreeHostCommonTest();
   explicit LayerTreeHostCommonTest(const LayerTreeSettings& settings);
+
+ protected:
+  static void SetScrollOffsetDelta(LayerImpl* layer_impl,
+                                   const gfx::Vector2dF& delta) {
+    layer_impl->SetCurrentScrollOffset(
+        layer_impl->synced_scroll_offset()->ActiveBase() +
+        gfx::ScrollOffset(delta));
+  }
 };
 
 }  // namespace cc
 
 #endif  // CC_TEST_LAYER_TREE_HOST_COMMON_TEST_H_