[heap] Reland uncommit unused large object page memory.

src/base/hashmap.h

Index: src/base/hashmap.h
diff --git a/src/base/hashmap.h b/src/base/hashmap.h
index efb5dc8baaafd7bdb0f036bca43347f1322aa42d..36755f62c3a051bf4bbe23d2a0515434594a0faf 100644
--- a/src/base/hashmap.h
+++ b/src/base/hashmap.h
@@ -61,6 +61,9 @@ class TemplateHashMapImpl {
   Entry* LookupOrInsert(void* key, uint32_t hash,
                         AllocationPolicy allocator = AllocationPolicy());
 
+  Entry* InsertNew(void* key, uint32_t hash,
+                   AllocationPolicy allocator = AllocationPolicy());
+
   // Removes the entry with matching key.
   // It returns the value of the deleted entry
   // or null if there is no value for such key.
@@ -134,6 +137,17 @@ TemplateHashMapImpl<AllocationPolicy>::LookupOrInsert(
     return p;
   }
 
+  return InsertNew(key, hash, allocator);
+}
+
+template <class AllocationPolicy>
+typename TemplateHashMapImpl<AllocationPolicy>::Entry*
+TemplateHashMapImpl<AllocationPolicy>::InsertNew(void* key, uint32_t hash,
+                                                 AllocationPolicy allocator) {
+  // Find a matching entry.
+  Entry* p = Probe(key, hash);
+  DCHECK(p->key == NULL);
+
   // No entry found; insert one.
   p->key = key;
   p->value = NULL;