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/task_unittest.cc b/webrtc/base/task_unittest.cc
index 38ceb14..b26d670 100644
--- a/webrtc/base/task_unittest.cc
+++ b/webrtc/base/task_unittest.cc
@@ -216,7 +216,7 @@
     for (i = 0; i < STUCK_TASK_COUNT; ++i) {
       if (stuck_[i].xlat_ == id) {
         stuck_[i].timed_out_ = true;
-        stuck_[i].task_ = NULL;
+        stuck_[i].task_ = nullptr;
         break;
       }
     }
@@ -231,7 +231,7 @@
     for (i = 0; i < HAPPY_TASK_COUNT; ++i) {
       if (happy_[i].xlat_ == id) {
         happy_[i].timed_out_ = true;
-        happy_[i].task_ = NULL;
+        happy_[i].task_ = nullptr;
         break;
       }
     }
@@ -245,7 +245,7 @@
     int i;
     for (i = 0; i < HAPPY_TASK_COUNT; ++i) {
       if (happy_[i].xlat_ == id) {
-        happy_[i].task_ = NULL;
+        happy_[i].task_ = nullptr;
         break;
       }
     }
@@ -347,7 +347,7 @@
   SetBoolOnDeleteTask(TaskParent *parent, bool *set_when_deleted)
     : Task(parent),
       set_when_deleted_(set_when_deleted) {
-    EXPECT_TRUE(NULL != set_when_deleted);
+    EXPECT_TRUE(nullptr != set_when_deleted);
     EXPECT_FALSE(*set_when_deleted);
   }
 
@@ -466,7 +466,7 @@
     for (size_t i = 0; i < arraysize(stuck_tasks_); ++i) {
       if (stuck_tasks_[i] && stuck_tasks_[i]->unique_id() == id) {
         task_count_--;
-        stuck_tasks_[i] = NULL;
+        stuck_tasks_[i] = nullptr;
         break;
       }
     }