Replace NULL with nullptr or null in webrtc/base/.

BUG=webrtc:7147

Review-Url: https://codereview.webrtc.org/2718663005
Cr-Commit-Position: refs/heads/master@{#16878}
diff --git a/webrtc/base/stringencode_unittest.cc b/webrtc/base/stringencode_unittest.cc
index fbd7975..60929f3 100644
--- a/webrtc/base/stringencode_unittest.cc
+++ b/webrtc/base/stringencode_unittest.cc
@@ -21,18 +21,18 @@
     size_t encsize, enclen;
     unsigned long decoded;
   } kTests[] = {
-    { "a    ",             5, 1, 'a' },
-    { "\x7F    ",          5, 1, 0x7F },
-    { "\xC2\x80   ",       5, 2, 0x80 },
-    { "\xDF\xBF   ",       5, 2, 0x7FF },
-    { "\xE0\xA0\x80  ",    5, 3, 0x800 },
-    { "\xEF\xBF\xBF  ",    5, 3, 0xFFFF },
-    { "\xF0\x90\x80\x80 ", 5, 4, 0x10000 },
-    { "\xF0\x90\x80\x80 ", 3, 0, 0x10000 },
-    { "\xF0\xF0\x80\x80 ", 5, 0, 0 },
-    { "\xF0\x90\x80  ",    5, 0, 0 },
-    { "\x90\x80\x80  ",    5, 0, 0 },
-    { NULL, 0, 0 },
+      {"a    ",             5, 1, 'a'},
+      {"\x7F    ",          5, 1, 0x7F},
+      {"\xC2\x80   ",       5, 2, 0x80},
+      {"\xDF\xBF   ",       5, 2, 0x7FF},
+      {"\xE0\xA0\x80  ",    5, 3, 0x800},
+      {"\xEF\xBF\xBF  ",    5, 3, 0xFFFF},
+      {"\xF0\x90\x80\x80 ", 5, 4, 0x10000},
+      {"\xF0\x90\x80\x80 ", 3, 0, 0x10000},
+      {"\xF0\xF0\x80\x80 ", 5, 0, 0},
+      {"\xF0\x90\x80  ",    5, 0, 0},
+      {"\x90\x80\x80  ",    5, 0, 0},
+      {nullptr, 0, 0},
   };
   for (size_t i = 0; kTests[i].encoded; ++i) {
     unsigned long val = 0;
@@ -248,7 +248,7 @@
 }
 
 TEST(TokenizeTest, TokenizeAppend) {
-  ASSERT_EQ(0ul, tokenize_append("A B C", ' ', NULL));
+  ASSERT_EQ(0ul, tokenize_append("A B C", ' ', nullptr));
 
   std::vector<std::string> fields;
 
@@ -263,7 +263,7 @@
 }
 
 TEST(TokenizeTest, TokenizeWithMarks) {
-  ASSERT_EQ(0ul, tokenize("D \"A B", ' ', '(', ')', NULL));
+  ASSERT_EQ(0ul, tokenize("D \"A B", ' ', '(', ')', nullptr));
 
   std::vector<std::string> fields;
   tokenize("A B C", ' ', '"', '"', &fields);