Replace NULL with nullptr in all C++ files.

webrtc/system_wrappers/source/aligned_malloc.cc

Index: webrtc/system_wrappers/source/aligned_malloc.cc
diff --git a/webrtc/system_wrappers/source/aligned_malloc.cc b/webrtc/system_wrappers/source/aligned_malloc.cc
index f700f8b1f19f967b8345b5688e88e50299af1f3d..8fd852c06bdda4fb7d56d50fd55982dd6875c909 100644
--- a/webrtc/system_wrappers/source/aligned_malloc.cc
+++ b/webrtc/system_wrappers/source/aligned_malloc.cc
@@ -41,10 +41,10 @@ bool ValidAlignment(size_t alignment) {
 
 void* GetRightAlign(const void* pointer, size_t alignment) {
   if (!pointer) {
-    return NULL;
+    return nullptr;
   }
   if (!ValidAlignment(alignment)) {
-    return NULL;
+    return nullptr;
   }
   uintptr_t start_pos = reinterpret_cast<uintptr_t>(pointer);
   return reinterpret_cast<void*>(GetRightAlign(start_pos, alignment));
@@ -52,10 +52,10 @@ void* GetRightAlign(const void* pointer, size_t alignment) {
 
 void* AlignedMalloc(size_t size, size_t alignment) {
   if (size == 0) {
-    return NULL;
+    return nullptr;
   }
   if (!ValidAlignment(alignment)) {
-    return NULL;
+    return nullptr;
   }
 
   // The memory is aligned towards the lowest address that so only
@@ -63,8 +63,8 @@ void* AlignedMalloc(size_t size, size_t alignment) {
   // A pointer to the start of the memory must be stored so that it can be
   // retreived for deletion, ergo the sizeof(uintptr_t).
   void* memory_pointer = malloc(size + sizeof(uintptr_t) + alignment - 1);
-  if (memory_pointer == NULL) {
-    return NULL;
+  if (memory_pointer == nullptr) {
+    return nullptr;
   }
 
   // Aligning after the sizeof(uintptr_t) bytes will leave room for the header
@@ -85,7 +85,7 @@ void* AlignedMalloc(size_t size, size_t alignment) {
 }
 
 void AlignedFree(void* mem_block) {
-  if (mem_block == NULL) {
+  if (mem_block == nullptr) {
     return;
   }
   uintptr_t aligned_pos = reinterpret_cast<uintptr_t>(mem_block);