Call SAC::DidScrollUpdate only for compositor-triggered scrolls.

cc/layers/scrollbar_layer_unittest.cc

 // Copyright 2012 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.
 
 #include <stddef.h>
 
 #include <unordered_map>
 
 #include "base/memory/ptr_util.h"
 #include "base/threading/thread_task_runner_handle.h"
@@ skipping 732 matching lines @@
 
   layer_tree_root->SetBounds(gfx::Size(2, 2));
   scroll_layer->SetBounds(gfx::Size(10, 10));
   layer_tree_host_->UpdateLayers();
   layer_tree_host_->CommitAndCreateLayerImplTree();
   LayerTreeHostImpl* host_impl = layer_tree_host_->host_impl();
   EXPECT_TRUE(host_impl->ScrollbarAnimationControllerForElementId(
       scroll_layer->element_id()));
 
   scroll_layer->SetBounds(gfx::Size(20, 20));
+  scroll_layer->ShowScrollbars();
   scroll_layer->SetForceRenderSurfaceForTesting(true);
   layer_tree_host_->UpdateLayers();
   host_impl->CreatePendingTree();
   layer_tree_host_->CommitAndCreatePendingTree();
   host_impl->ActivateSyncTree();
   EffectNode* node =
       host_impl->active_tree()->property_trees()->effect_tree.Node(
           scrollbar_layer->effect_tree_index());
   EXPECT_EQ(node->opacity, 1.f);
 }
@@ skipping 513 matching lines @@
   TestScale(gfx::Rect(1240, 0, 15, 1333), 2.7754839f);
   TestScale(gfx::Rect(1240, 0, 15, 677), 2.46677136f);
 
   // Horizontal Scrollbars.
   TestScale(gfx::Rect(0, 1240, 1333, 15), 2.7754839f);
   TestScale(gfx::Rect(0, 1240, 677, 15), 2.46677136f);
 }
 
 }  // namespace
 }  // namespace cc