Fix crash on win_aura buildbot because a RootWindow was getting multiple AddRootWindowObserver call…

content/browser/web_contents/web_contents_view_aura.cc

 // Copyright (c) 2012 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 "content/browser/web_contents/web_contents_view_aura.h"
 
 #include "base/auto_reset.h"
 #include "base/utf_string_conversions.h"
 #include "content/browser/renderer_host/dip_util.h"
 #include "content/browser/renderer_host/overscroll_controller.h"
@@ skipping 246 matching lines @@
     : public aura::WindowObserver, public aura::RootWindowObserver {
  public:
   explicit WindowObserver(WebContentsViewAura* view)
       : view_(view),
         parent_(NULL) {
     view_->window_->AddObserver(this);
   }
 
   virtual ~WindowObserver() {
     view_->window_->RemoveObserver(this);
+    if (view_->window_->GetRootWindow())
+      view_->window_->GetRootWindow()->RemoveRootWindowObserver(this);
     if (parent_)
       parent_->RemoveObserver(this);
   }
 
   // Overridden from aura::WindowObserver:
   virtual void OnWindowParentChanged(aura::Window* window,
                                      aura::Window* parent) OVERRIDE {
     if (window == parent_)
       return;
     if (parent_)
@@ skipping 717 matching lines @@
       event.location(),
       gfx::Screen::GetScreenFor(GetNativeView())->GetCursorScreenPoint(),
       ConvertAuraEventFlagsToWebInputEventModifiers(event.flags()));
   if (drag_dest_delegate_)
     drag_dest_delegate_->OnDrop();
   current_drop_data_.reset();
   return current_drag_op_;
 }
 
 }  // namespace content