Fix counting

Change-Id: I9a3568908a7223d69430c1765fb5a4db96d7ac9c
diff --git a/libs/hwui/Caches.cpp b/libs/hwui/Caches.cpp
index ad50894..952f739 100644
--- a/libs/hwui/Caches.cpp
+++ b/libs/hwui/Caches.cpp
@@ -280,7 +280,7 @@
                     layer->getWidth(), layer->getHeight(),
                     layer->isTextureLayer(), layer->getTexture(),
                     layer->getFbo(), layer->getStrongCount());
-            memused = layer->getWidth() * layer->getHeight() * 4;
+            memused += layer->getWidth() * layer->getHeight() * 4;
         }
         log.appendFormat("  Layers total   %8d (numLayers = %zu)\n",
                 memused, mRenderState->mActiveLayers.size());