Replace NULL with nullptr

Also fixed some lines that were too long, and a few other minor
details.

Change-Id: I6efba5fb6e03eb5d0a300fddb2a75bf8e2f175cb
diff --git a/runtime/mirror/stack_trace_element.cc b/runtime/mirror/stack_trace_element.cc
index ec2b495..96f6a53 100644
--- a/runtime/mirror/stack_trace_element.cc
+++ b/runtime/mirror/stack_trace_element.cc
@@ -30,7 +30,7 @@
 
 void StackTraceElement::SetClass(Class* java_lang_StackTraceElement) {
   CHECK(java_lang_StackTraceElement_.IsNull());
-  CHECK(java_lang_StackTraceElement != NULL);
+  CHECK(java_lang_StackTraceElement != nullptr);
   java_lang_StackTraceElement_ = GcRoot<Class>(java_lang_StackTraceElement);
 }
 
@@ -44,7 +44,7 @@
                                             int32_t line_number) {
   StackTraceElement* trace =
       down_cast<StackTraceElement*>(GetStackTraceElement()->AllocObject(self));
-  if (LIKELY(trace != NULL)) {
+  if (LIKELY(trace != nullptr)) {
     if (Runtime::Current()->IsActiveTransaction()) {
       trace->Init<true>(declaring_class, method_name, file_name, line_number);
     } else {