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/thread/TaskProcessor.h b/libs/hwui/thread/TaskProcessor.h
index d1269f0..eb4ab64 100644
--- a/libs/hwui/thread/TaskProcessor.h
+++ b/libs/hwui/thread/TaskProcessor.h
@@ -47,7 +47,7 @@
     virtual void onProcess(const sp<Task<T> >& task) = 0;
 
 private:
-    virtual void process(const sp<TaskBase>& task) {
+    virtual void process(const sp<TaskBase>& task) override {
         sp<Task<T> > realTask = static_cast<Task<T>* >(task.get());
         // This is the right way to do it but sp<> doesn't play nice
         // sp<Task<T> > realTask = static_cast<sp<Task<T> > >(task);