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/VertexBuffer.h b/libs/hwui/VertexBuffer.h
index 8c3a272..d81dd42 100644
--- a/libs/hwui/VertexBuffer.h
+++ b/libs/hwui/VertexBuffer.h
@@ -32,17 +32,17 @@
     };
 
     VertexBuffer()
-            : mBuffer(0)
-            , mIndices(0)
+            : mBuffer(nullptr)
+            , mIndices(nullptr)
             , mVertexCount(0)
             , mIndexCount(0)
             , mAllocatedVertexCount(0)
             , mAllocatedIndexCount(0)
             , mByteCount(0)
             , mMode(kStandard)
-            , mReallocBuffer(0)
-            , mCleanupMethod(NULL)
-            , mCleanupIndexMethod(NULL)
+            , mReallocBuffer(nullptr)
+            , mCleanupMethod(nullptr)
+            , mCleanupIndexMethod(nullptr)
     {}
 
     ~VertexBuffer() {