Fix bug in aggressive layer cache purging

src/gpu/GrLayerCache.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrLayerCache_DEFINED
 #define GrLayerCache_DEFINED
 
@@ skipping 12 matching lines @@
 
 // The layer cache listens for these messages to purge picture-related resources.
 struct GrPictureDeletedMessage {
     uint32_t pictureID;
 };
 
 // GrPictureInfo stores the atlas plots used by a single picture. A single 
 // plot may be used to store layers from multiple pictures.
 struct GrPictureInfo {
 public:
+    static const int kNumPlots = 4;
+
     // for SkTDynamicHash - just use the pictureID as the hash key
     static const uint32_t& GetKey(const GrPictureInfo& pictInfo) { return pictInfo.fPictureID; }
     static uint32_t Hash(const uint32_t& key) { return SkChecksum::Mix(key); }
 
     // GrPictureInfo proper
-    GrPictureInfo(uint32_t pictureID) : fPictureID(pictureID) { }
+    GrPictureInfo(uint32_t pictureID) : fPictureID(pictureID) { 
+#if !GR_CACHE_HOISTED_LAYERS
+        memset(fPlotUses, 0, sizeof(fPlotUses)); 
+#endif
+    }
+
+#if !GR_CACHE_HOISTED_LAYERS
+    void incPlotUsage(int plotID) {
+        SkASSERT(plotID < kNumPlots);
+        fPlotUses[plotID]++;
+    }
+
+    void decPlotUsage(int plotID) {
+        SkASSERT(plotID < kNumPlots);
+        SkASSERT(fPlotUses[plotID] > 0);
+        fPlotUses[plotID]--;
+    }
+
+    int plotUsage(int plotID) const { 
+        SkASSERT(plotID < kNumPlots);
+        return fPlotUses[plotID];
+    }
+#endif
 
     const uint32_t fPictureID;
+    GrAtlas::ClientPlotUsage  fPlotUsage;
 
-    GrAtlas::ClientPlotUsage  fPlotUsage;
+#if !GR_CACHE_HOISTED_LAYERS
+private:
+    int fPlotUses[kNumPlots];
+#endif
 };
 
 // GrCachedLayer encapsulates the caching information for a single saveLayer.
 //
 // Atlased layers get a ref to the backing GrTexture while non-atlased layers
 // get a ref to the GrTexture in which they reside. In both cases 'fRect' 
 // contains the layer's extent in its texture.
 // Atlased layers also get a pointer to the plot in which they reside.
 // For non-atlased layers, the lock field just corresponds to locking in
 // the resource cache. For atlased layers, it implements an additional level
@@ skipping 259 matching lines @@
         SkASSERT(fPlotLocks[plotIdx] > 0);
         --fPlotLocks[plotIdx];
     }
 
     // for testing
     friend class TestingAccess;
     int numLayers() const { return fLayerHash.count(); }
 };
 
 #endif