[heap] Introduce hard limit for external memory

src/heap/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 #include <map>
 
@@ skipping 823 matching lines @@
   void RememberUnmappedPage(Address page, bool compacted);
 
   // Global inline caching age: it is incremented on some GCs after context
   // disposal. We use it to flush inline caches.
   int global_ic_age() { return global_ic_age_; }
 
   void AgeInlineCaches() {
     global_ic_age_ = (global_ic_age_ + 1) & SharedFunctionInfo::ICAgeBits::kMax;
   }
 
+  int64_t external_memory_hard_limit() { return MaxOldGenerationSize() / 2; }
+
   int64_t external_memory() { return external_memory_; }
   void update_external_memory(int64_t delta) { external_memory_ += delta; }
 
   void update_external_memory_concurrently_freed(intptr_t freed) {
     external_memory_concurrently_freed_.Increment(freed);
   }
 
   void account_external_memory_concurrently_freed() {
     external_memory_ -= external_memory_concurrently_freed_.Value();
     external_memory_concurrently_freed_.SetValue(0);
@@ skipping 1873 matching lines @@
   friend class LargeObjectSpace;
   friend class NewSpace;
   friend class PagedSpace;
   DISALLOW_COPY_AND_ASSIGN(AllocationObserver);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_HEAP_H_