Ensure that pinch-zoom scrollbar animations trigger a draw

cc/layers/scrollbar_layer.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 "cc/layers/scrollbar_layer.h"
 
+#include "base/auto_reset.h"
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
 #include "cc/layers/scrollbar_layer_impl.h"
 #include "cc/resources/caching_bitmap_content_layer_updater.h"
 #include "cc/resources/layer_painter.h"
 #include "cc/resources/prioritized_resource.h"
 #include "cc/resources/resource_update_queue.h"
 #include "cc/trees/layer_tree_host.h"
 #include "third_party/WebKit/Source/Platform/chromium/public/WebRect.h"
 #include "ui/gfx/rect_conversions.h"
@@ skipping 376 matching lines @@
         ScrollbarLayerRectToContentRect(thumb_layer_rect).size();
     thumb_->texture()->SetDimensions(thumb_size, texture_format_);
     thumb_->texture()->set_request_priority(
         PriorityCalculator::UIPriority(draws_to_root));
   }
 }
 
 void ScrollbarLayer::Update(ResourceUpdateQueue* queue,
                             const OcclusionTracker* occlusion,
                             RenderingStats* stats) {
-  ContentsScalingLayer::Update(queue, occlusion, stats);
+  {
+    base::AutoReset<bool> ignore_set_needs_commit(&ignore_set_needs_commit_,
+                                                  true);
+    ContentsScalingLayer::Update(queue, occlusion, stats);
+  }
 
   dirty_rect_.Union(update_rect_);
   if (content_bounds().IsEmpty())
     return;
   if (visible_content_rect().IsEmpty())
     return;
 
   CreateUpdaterIfNeeded();
 
   gfx::Rect content_rect = ScrollbarLayerRectToContentRect(
@@ skipping 21 matching lines @@
                thumb_.get(),
                origin_thumb_rect,
                queue,
                stats);
   }
 
   dirty_rect_ = gfx::RectF();
 }
 
 }  // namespace cc