[runtime] Use nullptr instead of NULL when calling WRITE_FIELD.

BUG=chromium:694255
TBR=cbruni@chromium.org

Change-Id: I8a7907aa18d2855ca571ae564c7eeda41bf13fbb
Reviewed-on: https://chromium-review.googlesource.com/593655Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47017}

src/objects-inl.h

@@ -4937,15 +4937,15 @@ CODE_ACCESSORS(next_code_link, Object, kNextCodeLinkOffset)
 #undef CODE_ACCESSORS
 
 void Code::WipeOutHeader() {
-  WRITE_FIELD(this, kRelocationInfoOffset, NULL);
-  WRITE_FIELD(this, kHandlerTableOffset, NULL);
-  WRITE_FIELD(this, kDeoptimizationDataOffset, NULL);
-  WRITE_FIELD(this, kSourcePositionTableOffset, NULL);
+  WRITE_FIELD(this, kRelocationInfoOffset, nullptr);
+  WRITE_FIELD(this, kHandlerTableOffset, nullptr);
+  WRITE_FIELD(this, kDeoptimizationDataOffset, nullptr);
+  WRITE_FIELD(this, kSourcePositionTableOffset, nullptr);
   // Do not wipe out major/minor keys on a code stub or IC
   if (!READ_FIELD(this, kTypeFeedbackInfoOffset)->IsSmi()) {
-    WRITE_FIELD(this, kTypeFeedbackInfoOffset, NULL);
+    WRITE_FIELD(this, kTypeFeedbackInfoOffset, nullptr);
   }
-  WRITE_FIELD(this, kNextCodeLinkOffset, NULL);
+  WRITE_FIELD(this, kNextCodeLinkOffset, nullptr);
 }
 
 Object* Code::type_feedback_info() const {