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/PathCache.h b/libs/hwui/PathCache.h
index b0f00c7..ecd3712 100644
--- a/libs/hwui/PathCache.h
+++ b/libs/hwui/PathCache.h
@@ -87,7 +87,7 @@
     }
 
     void clearTask() {
-        if (mTask != NULL) {
+        if (mTask != nullptr) {
             mTask.clear();
         }
     }
@@ -165,7 +165,7 @@
      * Used as a callback when an entry is removed from the cache.
      * Do not invoke directly.
      */
-    void operator()(PathDescription& path, PathTexture*& texture);
+    void operator()(PathDescription& path, PathTexture*& texture) override;
 
     /**
      * Clears the cache. This causes all textures to be deleted.
@@ -292,7 +292,7 @@
         PathProcessor(Caches& caches);
         ~PathProcessor() { }
 
-        virtual void onProcess(const sp<Task<SkBitmap*> >& task);
+        virtual void onProcess(const sp<Task<SkBitmap*> >& task) override;
 
     private:
         uint32_t mMaxTextureSize;