Fix a crash when chooseDesktopMedia is called with no target tab

chrome/browser/extensions/api/desktop_capture/desktop_capture_apitest.cc

 // Copyright 2013 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 <queue>
 
 #include "base/command_line.h"
 #include "base/path_service.h"
 #include "base/thread_task_runner_handle.h"
 #include "chrome/browser/extensions/api/desktop_capture/desktop_capture_api.h"
@@ skipping 36 matching lines @@
   }
 
   // DesktopMediaPicker interface.
   virtual void Show(content::WebContents* web_contents,
                     gfx::NativeWindow context,
                     gfx::NativeWindow parent,
                     const base::string16& app_name,
                     const base::string16& target_name,
                     scoped_ptr<DesktopMediaList> model,
                     const DoneCallback& done_callback) OVERRIDE {
-    if (!expectation_->cancelled) {
+    EXPECT_TRUE(web_contents);
+    if (!expectation_->cancelled || !web_contents) {
       // Post a task to call the callback asynchronously.
       base::ThreadTaskRunnerHandle::Get()->PostTask(
           FROM_HERE,
           base::Bind(&FakeDesktopMediaPicker::CallCallback,
                      weak_factory_.GetWeakPtr(), done_callback));
     } else {
       // If we expect the dialog to be cancelled then store the callback to
       // retain reference to the callback handler.
       done_callback_ = done_callback;
     }
@@ skipping 166 matching lines @@
   EXPECT_TRUE(result);
   EXPECT_TRUE(test_flags[2].picker_created);
   EXPECT_FALSE(test_flags[2].picker_deleted);
 
   web_contents->Close();
   destroyed_watcher.Wait();
   EXPECT_TRUE(test_flags[2].picker_deleted);
 }
 
 }  // namespace extensions