Fix scrollbars missing after lost context

cc/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/scrollbar_layer.h"
 
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
 #include "cc/caching_bitmap_content_layer_updater.h"
 #include "cc/layer_painter.h"
@@ skipping 304 matching lines @@
 
 void ScrollbarLayer::update(ResourceUpdateQueue& queue, const OcclusionTracker* occlusion, RenderingStats& stats)
 {
     ContentsScalingLayer::update(queue, occlusion, stats);
 
     m_dirtyRect.Union(m_updateRect);
     if (contentBounds().IsEmpty())
         return;
     if (visibleContentRect().IsEmpty())
         return;
-    if (!isDirty())
-        return;
 
     createUpdaterIfNeeded();
 
     gfx::Rect contentRect = scrollbarLayerRectToContentRect(gfx::Rect(m_scrollbar->location(), bounds()));
     updatePart(m_backTrackUpdater.get(), m_backTrack.get(), contentRect, queue, stats);
     if (m_foreTrack && m_foreTrackUpdater)
         updatePart(m_foreTrackUpdater.get(), m_foreTrack.get(), contentRect, queue, stats);
 
     // Consider the thumb to be at the origin when painting.
     gfx::Rect thumbRect = m_geometry->thumbRect(m_scrollbar.get());
     gfx::Rect originThumbRect = scrollbarLayerRectToContentRect(gfx::Rect(thumbRect.size()));
     if (!originThumbRect.IsEmpty())
         updatePart(m_thumbUpdater.get(), m_thumb.get(), originThumbRect, queue, stats);
 
     m_dirtyRect = gfx::RectF();
 }
 
 }  // namespace cc