Add additional clean up code in MPD path

This addresses an edge case where a picture only has one layer which is atlasable but doesn't make it into the atlas. In this case asserts can fire since there is no atlased layer to clean up the tracking picture object.

Review URL: https://codereview.chromium.org/764393002

src/gpu/GrLayerCache.cpp

@@ -226,6 +226,11 @@ bool GrLayerCache::tryToAtlas(GrCachedLayer* layer,
                 break;  // We weren't able to purge any plots
             }
         }
+
+        if (pictInfo->fPlotUsage.isEmpty()) {
+            fPictureHash.remove(pictInfo->fPictureID);
+            SkDELETE(pictInfo);
+        }
     }
 
     return false;