Fix a crash which occurs while dragging tabs back to the window where they originated from.

ui/views/controls/webview/webview_unittest.cc

 // Copyright 2014 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 "ui/views/controls/webview/webview.h"
 
 #include "base/memory/scoped_ptr.h"
 #include "content/browser/web_contents/web_contents_impl.h"
 #include "content/public/browser/web_contents_observer.h"
 #include "content/public/test/test_browser_context.h"
@@ skipping 84 matching lines @@
       content::WebContentsObserver::Observe(NULL);
   }
 
   virtual void WebContentsDestroyed() OVERRIDE {
     DCHECK(web_contents_);
     content::WebContentsObserver::Observe(NULL);
     web_contents_ = NULL;
   }
 
   virtual void WasShown() OVERRIDE {
+    valid_root_while_shown_ =
+        web_contents()->GetNativeView()->GetRootWindow() != NULL;
     was_shown_ = true;
     ++shown_count_;
   }
 
   virtual void WasHidden() OVERRIDE {
     was_shown_ = false;
     ++hidden_count_;
   }
 
   bool was_shown() const { return was_shown_; }
 
   int shown_count() const { return shown_count_; }
 
   int hidden_count() const { return hidden_count_; }
 
+  bool valid_root_while_shown() const { return valid_root_while_shown_; }
+
  private:
   content::WebContentsImpl* web_contents_;
   bool was_shown_;
   int32 shown_count_;
   int32 hidden_count_;
+  // Set to true if the view containing the webcontents has a valid root window.
+  bool valid_root_while_shown_;
 
   DISALLOW_COPY_AND_ASSIGN(WebViewTestWebContentsObserver);
 };
 
 // Tests that attaching and detaching a WebContents to a WebView makes the
 // WebContents visible and hidden respectively.
 TEST_F(WebViewUnitTest, TestWebViewAttachDetachWebContents) {
   // Create a top level widget and a webview as its content.
   views::Widget* widget = CreateTopLevelFramelessPlatformWidget();
   widget->SetBounds(gfx::Rect(0, 10, 100, 100));
   views::WebView* webview = new views::WebView(browser_context());
   widget->SetContentsView(webview);
   widget->Show();
 
   // Case 1: Create a new WebContents and set it in the webview via
   // SetWebContents. This should make the WebContents visible.
   content::WebContents::CreateParams params(browser_context());
   scoped_ptr<content::WebContents> web_contents1(
       content::WebContents::Create(params));
   WebViewTestWebContentsObserver observer1(web_contents1.get());
   EXPECT_FALSE(observer1.was_shown());
 
   webview->SetWebContents(web_contents1.get());
   EXPECT_TRUE(observer1.was_shown());
   EXPECT_TRUE(web_contents1->GetNativeView()->IsVisible());
   EXPECT_EQ(observer1.shown_count(), 1);
   EXPECT_EQ(observer1.hidden_count(), 0);
+  EXPECT_TRUE(observer1.valid_root_while_shown());
 
   // Case 2: Create another WebContents and replace the current WebContents
   // via SetWebContents(). This should hide the current WebContents and show
   // the new one.
   content::WebContents::CreateParams params2(browser_context());
   scoped_ptr<content::WebContents> web_contents2(
       content::WebContents::Create(params2));
+
   WebViewTestWebContentsObserver observer2(web_contents2.get());
   EXPECT_FALSE(observer2.was_shown());
 
   // Setting the new WebContents should hide the existing one.
   webview->SetWebContents(web_contents2.get());
   EXPECT_FALSE(observer1.was_shown());
   EXPECT_TRUE(observer2.was_shown());
+  EXPECT_TRUE(observer2.valid_root_while_shown());
 
   // WebContents1 should not get stray show calls when WebContents2 is set.
   EXPECT_EQ(observer1.shown_count(), 1);
   EXPECT_EQ(observer1.hidden_count(), 1);
   EXPECT_EQ(observer2.shown_count(), 1);
   EXPECT_EQ(observer2.hidden_count(), 0);
 
   widget->Close();
   RunPendingMessages();
 }
 
 }  // namespace