Revert of [heap] Decouple old generation allocation limit from external memory.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 624744f7ee02e495a9fb8ea281005cce0765911b..101c25fd180b3fc68ebb6f8b09e381070464c3d1 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1352,6 +1352,16 @@
     survived_since_last_expansion_ += survived;
   }
 
+  inline intptr_t PromotedTotalSize() {
+    int64_t total = PromotedSpaceSizeOfObjects() + PromotedExternalMemorySize();
+    if (total > std::numeric_limits<intptr_t>::max()) {
+      // TODO(erikcorry): Use uintptr_t everywhere we do heap size calculations.
+      return std::numeric_limits<intptr_t>::max();
+    }
+    if (total < 0) return 0;
+    return static_cast<intptr_t>(total);
+  }
+
   inline void UpdateNewSpaceAllocationCounter();
 
   inline size_t NewSpaceAllocationCounter();
@@ -1794,7 +1804,7 @@
   // ===========================================================================
 
   inline intptr_t OldGenerationSpaceAvailable() {
-    return old_generation_allocation_limit_ - PromotedSpaceSizeOfObjects();
+    return old_generation_allocation_limit_ - PromotedTotalSize();
   }
 
   // Returns maximum GC pause.