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/base/mutex.h b/runtime/base/mutex.h
index 6e4b96c..f2be85e 100644
--- a/runtime/base/mutex.h
+++ b/runtime/base/mutex.h
@@ -344,8 +344,8 @@
   // Assert the current thread has shared access to the ReaderWriterMutex.
   void AssertSharedHeld(const Thread* self) {
     if (kDebugLocking && (gAborting == 0)) {
-      // TODO: we can only assert this well when self != NULL.
-      CHECK(IsSharedHeld(self) || self == NULL) << *this;
+      // TODO: we can only assert this well when self != null.
+      CHECK(IsSharedHeld(self) || self == nullptr) << *this;
     }
   }
   void AssertReaderHeld(const Thread* self) { AssertSharedHeld(self); }