cc: Don't use partial updates for scrollbars when they are not allowed.

cc/scrollbar_layer.h

Index: cc/scrollbar_layer.h
diff --git a/cc/scrollbar_layer.h b/cc/scrollbar_layer.h
index 2ddbad7b41eb78af13e354c7c23dc3f6032a17cb..1816f51ececb0fe87b55d2ffb4be74a32bd45ebb 100644
--- a/cc/scrollbar_layer.h
+++ b/cc/scrollbar_layer.h
@@ -58,6 +58,8 @@ private:
     void createUpdaterIfNeeded();
     gfx::Rect scrollbarLayerRectToContentRect(const gfx::Rect& layerRect) const;
 
+    bool isDirty() const { return !m_dirtyRect.IsEmpty(); }
+
     int maxTextureSize();
     float clampScaleToMaxTextureSize(float scale);
 
@@ -68,6 +70,8 @@ private:
 
     GLenum m_textureFormat;
 
+    gfx::RectF m_dirtyRect;
+
     scoped_refptr<CachingBitmapContentLayerUpdater> m_backTrackUpdater;
     scoped_refptr<CachingBitmapContentLayerUpdater> m_foreTrackUpdater;
     scoped_refptr<CachingBitmapContentLayerUpdater> m_thumbUpdater;