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/RevealClip.h b/libs/hwui/RevealClip.h
index a9600f1..0084a8e 100644
--- a/libs/hwui/RevealClip.h
+++ b/libs/hwui/RevealClip.h
@@ -54,7 +54,7 @@
     float getRadius() const { return mRadius; }
 
     const SkPath* getPath() const {
-        if (!mShouldClip) return NULL;
+        if (!mShouldClip) return nullptr;
 
         return &mPath;
     }