Clear input event router entries for destroyed RWHVGuests

content/browser/frame_host/render_widget_host_view_guest.cc

Index: content/browser/frame_host/render_widget_host_view_guest.cc
diff --git a/content/browser/frame_host/render_widget_host_view_guest.cc b/content/browser/frame_host/render_widget_host_view_guest.cc
index 6d4a1811c5bc9eae2ed516d37e990a25d42bc905..3c7361afdb677428d00a00800acd8ac18ef1c82a 100644
--- a/content/browser/frame_host/render_widget_host_view_guest.cc
+++ b/content/browser/frame_host/render_widget_host_view_guest.cc
@@ -558,6 +558,7 @@ gfx::NativeViewId RenderWidgetHostViewGuest::GetParentForWindowlessPlugin()
 #endif
 
 void RenderWidgetHostViewGuest::DestroyGuestView() {
+  UnregisterSurfaceNamespaceId();

[wjmaclean, 2016/01/27 21:39:28]

So this eventually calls back to  WebContentsImpl::GetInputEventRouter() which
will give one of two answers, depending on whether the Guest WebContents has
been 'detached' from the owner WebContents (GetOuterWebContents()).

Since this also gets called from BrowserPluginGuest::OnDetach() we can hope that
GetOuterWebContents() is still non-null in the crash case, ** so this is worth
trying **. But if the crashes continue to occur, we should next try to check to
see if the owner has gone away unexpectedly somehow ...

   host_->SetView(NULL);
   host_ = NULL;
   base::MessageLoop::current()->DeleteSoon(FROM_HERE, this);