Return Object* instead of MaybeObject* from runtime calls.

src/serialize.cc

Index: src/serialize.cc
diff --git a/src/serialize.cc b/src/serialize.cc
index a94181c1d85d3005b4f4a44e8a69c26bede130b1..64b1c96ebf5c881fb9be8286c6f1fd250035bd0d 100644
--- a/src/serialize.cc
+++ b/src/serialize.cc
@@ -320,15 +320,6 @@ void ExternalReferenceTable::PopulateTable(Isolate* isolate) {
       "StubCache::secondary_->map");
 
   // Runtime entries
-  Add(ExternalReference::perform_gc_function(isolate).address(),
-      RUNTIME_ENTRY,
-      1,
-      "Runtime::PerformGC");
-  // Runtime entries
-  Add(ExternalReference::out_of_memory_function(isolate).address(),
-      RUNTIME_ENTRY,
-      2,
-      "Runtime::OutOfMemory");
   Add(ExternalReference::delete_handle_scope_extensions(isolate).address(),
       RUNTIME_ENTRY,
       4,
@@ -383,10 +374,6 @@ void ExternalReferenceTable::PopulateTable(Isolate* isolate) {
       UNCLASSIFIED,
       11,
       "Heap::NewSpaceMask()");
-  Add(ExternalReference::heap_always_allocate_scope_depth(isolate).address(),
-      UNCLASSIFIED,
-      12,
-      "Heap::always_allocate_scope_depth()");
   Add(ExternalReference::new_space_allocation_limit_address(isolate).address(),
       UNCLASSIFIED,
       14,