v8binding: Fixes wrapper tracing to follow: LocalDOMWindow => Document.

The root cause of https://crbug.com/745771 is that a listener object,
stored in V8AbstractEventListener::listener_, is wrongly collected by
V8 GC.

Listener objects are expected to be wrapper-traced at
DEFINE_TRACE_WRAPPERS(EventTarget) through
DEFINE_TRACE_WRAPPERS(Node) and DEFINE_TRACE_WRAPPERS(ContainerNode),
however, there seems no root object which kicks this path of wrapper-
tracing.

This CL adds a new path of wrapper-tracing from LocalDOMWindow to
Document.  Since LocalDOMWindow is a root object and Document is a
ContainerNode which wrapper-traces all child nodes, this CL makes sure
that all event listeners registered at any part of the document tree
alive.  (If an EventTarget is not attached to the DOM, such a case is
not covered by this CL.)

The fix was confirmed manually with using a test Chrome extension.

Bug: 745771
Change-Id: I7c0d1fb766b78fd4d32ce813662c947ce5f7e819
Reviewed-on: https://chromium-review.googlesource.com/591728Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490742}

third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp

@@ -249,6 +249,7 @@ static void UntrackAllBeforeUnloadEventListeners(LocalDOMWindow* dom_window) {
 
 LocalDOMWindow::LocalDOMWindow(LocalFrame& frame)
     : DOMWindow(frame),
+      document_(this, nullptr),
       visualViewport_(DOMVisualViewport::Create(this)),
       unused_preloads_timer_(
           TaskRunnerHelper::Get(TaskType::kUnspecedTimer, &frame),
@@ -1646,6 +1647,7 @@ DEFINE_TRACE(LocalDOMWindow) {
 
 DEFINE_TRACE_WRAPPERS(LocalDOMWindow) {
   visitor->TraceWrappers(custom_elements_);
+  visitor->TraceWrappers(document_);
   visitor->TraceWrappers(modulator_);
   DOMWindow::TraceWrappers(visitor);
 }

third_party/WebKit/Source/core/frame/LocalDOMWindow.h

@@ -340,7 +340,7 @@ class CORE_EXPORT LocalDOMWindow final : public DOMWindow,
   void DispatchLoadEvent();
   void ClearDocument();
 
-  Member<Document> document_;
+  TraceWrapperMember<Document> document_;
   Member<DOMVisualViewport> visualViewport_;
   TaskRunnerTimer<LocalDOMWindow> unused_preloads_timer_;