base: Use nullptr instead of NULL and static_cast in discardable memory unit tests.

BUG=422953
TEST=base_unittests --gtest_filter=DiscardableMemoryTests*
TBR=danakj@chromium.org

Review URL: https://codereview.chromium.org/659143002

Cr-Commit-Position: refs/heads/master@{#299991}

base/memory/discardable_memory_unittest.cc

@@ -65,13 +65,13 @@ TEST_P(DiscardableMemoryTest, LockAndUnLock) {
   const scoped_ptr<DiscardableMemory> memory(CreateLockedMemory(kSize));
   ASSERT_TRUE(memory);
   void* addr = memory->Memory();
-  ASSERT_NE(static_cast<void*>(NULL), addr);
+  ASSERT_NE(nullptr, addr);
 
   memory->Unlock();
 
   EXPECT_NE(DISCARDABLE_MEMORY_LOCK_STATUS_FAILED, memory->Lock());
   addr = memory->Memory();
-  ASSERT_NE(static_cast<void*>(NULL), addr);
+  ASSERT_NE(nullptr, addr);
 
   memory->Unlock();
 }