Fix missing case for JS_PROMISE_TYPE for header size.

TBR=verwaest@chromium.org
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/1459253002

Cr-Commit-Position: refs/heads/master@{#32122}

src/objects-inl.h

@@ -2120,6 +2120,8 @@ int JSObject::GetHeaderSize(InstanceType type) {
       return JSWeakMap::kSize;
     case JS_WEAK_SET_TYPE:
       return JSWeakSet::kSize;
+    case JS_PROMISE_TYPE:
+      return JSObject::kHeaderSize;
     case JS_REGEXP_TYPE:
       return JSRegExp::kSize;
     case JS_CONTEXT_EXTENSION_OBJECT_TYPE: