[heap-profiler] Make WeakMap key edge name more descriptive.

Currently a WeakMap key edge shows up as '<id> / WeakMap'. This patch
changes it to '<id> / key <KeyName> in WeakMap'.

Bug: chromium:827713
Change-Id: I0306bacdc331ab69739be88cf29d16a21187625f
Reviewed-on: https://chromium-review.googlesource.com/992035Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52465}

src/profiler/heap-snapshot-generator.cc

@@ -1364,8 +1364,10 @@ void V8HeapExplorer::ExtractFixedArrayReferences(int entry, FixedArray* array) {
         int key_entry_index = key_entry->index();
         HeapEntry* value_entry = GetEntry(value);
         if (key_entry && value_entry) {
+          const char* edge_name =
+              names_->GetFormatted("key %s in WeakMap", key_entry->name());
           filler_->SetNamedAutoIndexReference(HeapGraphEdge::kInternal,
-                                              key_entry_index, "WeakMap",
+                                              key_entry_index, edge_name,
                                               value_entry);
         }
       }

test/cctest/test-heap-profiler.cc

@@ -768,7 +768,7 @@ TEST(HeapSnapshotEphemeron) {
     const v8::HeapGraphNode* child = edge->GetToNode();
     if (!strcmp("ValueClass", GetName(child))) {
       v8::String::Utf8Value edge_name(CcTest::isolate(), edge->GetName());
-      CHECK(EndsWith(*edge_name, " / WeakMap"));
+      CHECK(EndsWith(*edge_name, " / key KeyClass in WeakMap"));
       success = true;
       break;
     }