Reland "[heap-profiler] Do not treat WeakMap values as weak."

This is a reland of c34042cc
Original change's description:
> [heap-profiler] Do not treat WeakMap values as weak.
> 
> For the WeakHashTable objects only mark keys as weak while leaving values as strong references.
> 
> BUG=chomium:773722
> 
> Change-Id: Iabd5ba293d05fe68a2af6503fcdd711ecc182482
> Reviewed-on: https://chromium-review.googlesource.com/730771
> Commit-Queue: Alexei Filippov <alph@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48985}

TBR=ulan@chromium.org

Bug: chomium:773722
Change-Id: Icede16fde528d147cde5c3f6c72f2029876b099f
Reviewed-on: https://chromium-review.googlesource.com/740722Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48987}

src/objects/hash-table.h

@@ -597,8 +597,6 @@ class WeakHashTable : public HashTable<WeakHashTable, WeakHashTableShape<2>> {
                                                    Handle<HeapObject> key,
                                                    Handle<HeapObject> value);
 
-  static Handle<FixedArray> GetValues(Handle<WeakHashTable> table);
-
  private:
   friend class MarkCompactCollector;
 

src/profiler/heap-snapshot-generator.cc

@@ -1345,12 +1345,21 @@ void V8HeapExplorer::ExtractFixedArrayReferences(int entry, FixedArray* array) {
     return;
   }
   switch (it->second) {
-    case JS_WEAK_COLLECTION_SUB_TYPE:
-      for (int i = 0, l = array->length(); i < l; ++i) {
-        SetWeakReference(array, entry, i, array->get(i),
-                         array->OffsetOfElementAt(i));
+    case JS_WEAK_COLLECTION_SUB_TYPE: {
+      ObjectHashTable* table = ObjectHashTable::cast(array);
+      for (int i = 0, capacity = table->Capacity(); i < capacity; ++i) {
+        int key_index =
+            ObjectHashTable::EntryToIndex(i) + ObjectHashTable::kEntryKeyIndex;
+        int value_index = ObjectHashTable::EntryToValueIndex(i);
+        SetWeakReference(table, entry, key_index, table->get(key_index),
+                         table->OffsetOfElementAt(key_index));
+        SetInternalReference(table, entry, value_index, table->get(value_index),
+                             table->OffsetOfElementAt(value_index));
+        // TODO(alph): Add a strong link (shortcut?) from key to value per
+        //             WeakMap the key was added to. See crbug.com/778739
       }
       break;
+    }
 
     // TODO(alph): Add special processing for other types of FixedArrays.
 

test/cctest/test-heap-profiler.cc

@@ -558,7 +558,7 @@ TEST(HeapSnapshotWeakCollection) {
       ++weak_entries;
     }
   }
-  CHECK_EQ(2, weak_entries);
+  CHECK_EQ(1, weak_entries);  // Key is the only weak.
   const v8::HeapGraphNode* wm_s =
       GetProperty(env->GetIsolate(), wm, v8::HeapGraphEdge::kProperty, "str");
   CHECK(wm_s);