Presentation API: ignore page visibility changes if ExecutionContext is destoyed.

Source/platform/exported/WebPresentationAvailabilityObserver.cpp

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "config.h"
+#include "public/platform/modules/presentation/WebPresentationAvailabilityObserver.h"
+
+// This WebPresentationAvailabilityObserver.cpp, which includes only config.h
+// and WebPresentationAvailabilityObserver.h, should be in
+// Source/platform/exported,  because WebPresentationAvailabilityObserver is not
+// compiled without this cpp. So if we don't have this cpp, we will see
+// unresolved symbol error when constructor/destructor's address is required.

[mark a. foltz, 2015/09/08 20:01:12]

I didn't quite follow this comment or the problem this is trying to solve.

I am not sure anything in Blink should be taking the address of the default
ctor/dtor, as these functions will be overridden by the embedder.  If you want
to get a handle to the real ctor/dtor, it seems like you need to implement a
wrapper that indirects through a pointer (and thus the vtable), similar to
struct DefaultDeleter in scoped_ptr.

At any rate, the module including .h should be able to take the address of the
default ctor/dtors declared there - as there is nothing defined in this cpp.

Maybe there is some linker magic that I'm missing :)

[mlamouri (slow - plz ping), 2015/09/08 21:53:59]

This is coming from WebPresentationController.cpp which was created because of
recents changes coming from componetization. Without that file, some compilers
(ie. Windows) will not find the definition of ctor and dtor for
WebPresentationController.