Add overrides and switch to nullptr keyword

Changes generated with clang-modernize.

Additionally, fixed some struct-vs-class usage to make clang happy.

Change-Id: Ic6ef2427401ff1e794d26f21f7b44868fc75fb72
diff --git a/libs/hwui/TessellationCache.h b/libs/hwui/TessellationCache.h
index a7c50eb..3efeaf6 100644
--- a/libs/hwui/TessellationCache.h
+++ b/libs/hwui/TessellationCache.h
@@ -166,7 +166,7 @@
     sp<TaskProcessor<VertexBuffer*> > mProcessor;
     LruCache<Description, Buffer*> mCache;
     class BufferRemovedListener : public OnEntryRemoved<Description, Buffer*> {
-        void operator()(Description& description, Buffer*& buffer);
+        void operator()(Description& description, Buffer*& buffer) override;
     };
     BufferRemovedListener mBufferRemovedListener;
 
@@ -178,8 +178,8 @@
     // holds a pointer, and implicit strong ref to each shadow task of the frame
     LruCache<ShadowDescription, Task<vertexBuffer_pair_t*>*> mShadowCache;
     class BufferPairRemovedListener : public OnEntryRemoved<ShadowDescription, Task<vertexBuffer_pair_t*>*> {
-        void operator()(ShadowDescription& description, Task<vertexBuffer_pair_t*>*& bufferPairTask) {
-            bufferPairTask->decStrong(NULL);
+        void operator()(ShadowDescription& description, Task<vertexBuffer_pair_t*>*& bufferPairTask) override {
+            bufferPairTask->decStrong(nullptr);
         }
     };
     BufferPairRemovedListener mBufferPairRemovedListener;