[wasm] explicitly mark off unlinked wasm module instances

This fixes a gc stress bug. We cannot rely on an ordering of
clearing of the weak cells, so we explicitly reset the weak
link to the owning instance, when finalizing a compiled
module. In turn, this serves as a reliable signal when GCs
happen while instantiating, allowing us to correctly link the
new instance.

BUG=chromium:652425

Review-Url: https://codereview.chromium.org/2393443003
Cr-Commit-Position: refs/heads/master@{#39964}

src/wasm/wasm-module.cc

@@ -773,7 +773,6 @@ static void ResetCompiledModule(Isolate* isolate, JSObject* owner,
       }
     }
   }
-  compiled_module->reset_weak_owning_instance();
   compiled_module->reset_heap();
 }
 
@@ -831,6 +830,7 @@ static void InstanceFinalizer(const v8::WeakCallbackInfo<void>& data) {
       }
     }
   }
+  compiled_module->reset_weak_owning_instance();
   GlobalHandles::Destroy(reinterpret_cast<Object**>(p));
 }