[tab_capture] Change LOG(DFATAL) to LOG(WARNING) if fail to find new parents for offscreen tab.

chrome/browser/extensions/api/tab_capture/tab_capture_registry.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 "chrome/browser/extensions/api/tab_capture/tab_capture_registry.h"
 
 #include <utility>
 
 #include "base/lazy_instance.h"
 #include "base/macros.h"
@@ skipping 86 matching lines @@
         browsers ? browsers->GetLastActive() : nullptr;
     BrowserWindow* const active_window =
         active_browser ? active_browser->window() : nullptr;
     aura::Window* const native_window =
         active_window ? active_window->GetNativeWindow() : nullptr;
     aura::Window* const root_window =
         native_window ? native_window->GetRootWindow() : nullptr;
     if (root_window) {
       DVLOG(2) << "Root window " << root_window
                << " adopts the content window " << content_window_ << '.';
       root_window->AddChild(content_window_);

[miu, 2017/02/24 22:51:55]

Some work has been done in content::DelegatedFrameHost, where capture now comes
directly from the Surface. This may mean we don't need this WindowAdoptionAgent
hack anymore.

So, on your desktop, can you comment out this LOC; and then run the
browser_tests to confirm whether offscreen tabs still render and capture?

  ninja -C out/dir browser_tests
  browser_tests --gtest_filter='*TabCaptureApiPixelTest*Offscreen*'

If they still work, let's simply delete the WindowAdoptionAgent class. :)

[zhaobin, 2017/02/25 00:30:44]

browser_tests passed if comment out this LOC. But when trying to start an
offscreen presentation, my cast device shows a green screen. It seems that
WindowAdoptionAgent is still needed.

     } else {
-      LOG(DFATAL) << "Unable to find an aura root window.  "
-                     "Compositing of the content may be halted!";
+      LOG(WARNING) << "Unable to find an aura root window.  "
+                      "Compositing of the content may be halted!";
     }
   }
 
   aura::Window* content_window_;
   base::WeakPtrFactory<WindowAdoptionAgent> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowAdoptionAgent);
 };
 #endif  // USE_AURA
 
@@ skipping 343 matching lines @@
        it != requests_.end(); ++it) {
     if (it->get() == request) {
       requests_.erase(it);
       return;
     }
   }
   NOTREACHED();
 }
 
 }  // namespace extensions