ui::Compositor needs a valid FrameSinkId even when it doesn't have ContextFactoryPrivate.

ui/compositor/compositor.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 "ui/compositor/compositor.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <deque>
@@ skipping 67 matching lines @@
       context_factory_private_(context_factory_private),
       root_layer_(NULL),
       widget_(gfx::kNullAcceleratedWidget),
 #if defined(USE_AURA)
       window_(nullptr),
 #endif
       widget_valid_(false),
       compositor_frame_sink_requested_(false),
       frame_sink_id_(context_factory_private
                          ? context_factory_private->AllocateFrameSinkId()
-                         : cc::FrameSinkId()),
+                         : cc::FrameSinkId(1, 1)),

[sadrul, 2016/12/16 02:29:09]

Let's have a kDefaultFrameSinkId for this. So something like:

  constexpr cc::FrameSinkId kDefaultFrameSinkId(1, 1);

up in the anon namespace. And add a comment explaining why we need a valid
sink-id here.

[mfomitchev, 2016/12/16 18:17:58]

Done.

       task_runner_(task_runner),
       vsync_manager_(new CompositorVSyncManager()),
       device_scale_factor_(0.0f),
       locks_will_time_out_(true),
       compositor_lock_(NULL),
       layer_animator_collection_(this),
       weak_ptr_factory_(this) {
   if (context_factory_private) {
     context_factory_private->GetSurfaceManager()->RegisterFrameSinkId(
         frame_sink_id_);
@@ skipping 459 matching lines @@
   for (auto& observer : observer_list_)
     observer.OnCompositingLockStateChanged(this);
 }
 
 void Compositor::CancelCompositorLock() {
   if (compositor_lock_)
     compositor_lock_->CancelLock();
 }
 
 }  // namespace ui