Rename NullHandle to ScopedNullHandle

This makes it clearer that is invalid to do things like:
Handle<T> h = ScopedNullHandle<T>();

Bug: 26233305
Change-Id: I6d8f54eae01ec2e901cb7043afa853ea77db79fe
diff --git a/compiler/optimizing/gvn_test.cc b/compiler/optimizing/gvn_test.cc
index de60cf2..78cb7d4 100644
--- a/compiler/optimizing/gvn_test.cc
+++ b/compiler/optimizing/gvn_test.cc
@@ -28,7 +28,7 @@
 TEST(GVNTest, LocalFieldElimination) {
   ArenaPool pool;
   ArenaAllocator allocator(&pool);
-  NullHandle<mirror::DexCache> dex_cache;
+  ScopedNullHandle<mirror::DexCache> dex_cache;
 
   HGraph* graph = CreateGraph(&allocator);
   HBasicBlock* entry = new (&allocator) HBasicBlock(graph);
@@ -113,7 +113,7 @@
 TEST(GVNTest, GlobalFieldElimination) {
   ArenaPool pool;
   ArenaAllocator allocator(&pool);
-  NullHandle<mirror::DexCache> dex_cache;
+  ScopedNullHandle<mirror::DexCache> dex_cache;
 
   HGraph* graph = CreateGraph(&allocator);
   HBasicBlock* entry = new (&allocator) HBasicBlock(graph);
@@ -196,7 +196,7 @@
 TEST(GVNTest, LoopFieldElimination) {
   ArenaPool pool;
   ArenaAllocator allocator(&pool);
-  NullHandle<mirror::DexCache> dex_cache;
+  ScopedNullHandle<mirror::DexCache> dex_cache;
 
   HGraph* graph = CreateGraph(&allocator);
   HBasicBlock* entry = new (&allocator) HBasicBlock(graph);
@@ -319,7 +319,7 @@
 TEST(GVNTest, LoopSideEffects) {
   ArenaPool pool;
   ArenaAllocator allocator(&pool);
-  NullHandle<mirror::DexCache> dex_cache;
+  ScopedNullHandle<mirror::DexCache> dex_cache;
 
   static const SideEffects kCanTriggerGC = SideEffects::CanTriggerGC();