WIP - Gutterless resize on Windows

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index fbc801dbe7cd13b8708985c76eb565c09859e038..57b886e69a14cd3296349fe922061195ad3450c3 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -79,6 +79,7 @@ Compositor::Compositor(ui::ContextFactory* context_factory,
       locks_will_time_out_(true),
       compositor_lock_(NULL),
       layer_animator_collection_(this),
+      has_swapped_frame_at_current_size_(false),
       weak_ptr_factory_(this) {
   root_web_layer_ = cc::Layer::Create(Layer::UILayerSettings());
 
@@ -277,6 +278,10 @@ void Compositor::SetLatencyInfo(const ui::LatencyInfo& latency_info) {
 void Compositor::SetScaleAndSize(float scale, const gfx::Size& size_in_pixel) {
   DCHECK_GT(scale, 0);
   if (!size_in_pixel.IsEmpty()) {
+    if (size_ != size_in_pixel) {
+      has_swapped_frame_at_current_size_ = false;
+    }
+
     size_ = size_in_pixel;
     host_->SetViewportSize(size_in_pixel);
     root_web_layer_->SetBounds(size_in_pixel);
@@ -438,6 +443,7 @@ void Compositor::DidCommitAndDrawFrame() {
 }
 
 void Compositor::DidCompleteSwapBuffers() {
+  has_swapped_frame_at_current_size_ = true;

[piman, 2015/12/11 07:53:12]

It is possible that the main thread will start the next frame before it received
the SwapBuffers ack (e.g. if the GPU process is under load). If that happens, it
could be that SetScaleAndSize is called again before this is called, in which
case it wouldn't be appropriate to set this to true. We could also have multiple
frames in the pipeline.

You'd want to track the size as it goes through the compositor steps. After you
set the size you need all of these to happen:
- commit
- draw frame
- post swap
- complete swap

you only got a "correct" frame if SetScaleAndSize wasn't called between any of
these steps. You can either track that manually (e.g. have a state enum, that
gets advanced in each of the relevant callbacks, and reset in SetScaleAndSize),
or using a SwapPromise which will track the first 3 for you.

[ericrk, 2015/12/15 00:25:03]

Good feedback - I've updated this to use a SwapPromise to track most of this.
Let me know how the new version looks. 

   FOR_EACH_OBSERVER(CompositorObserver, observer_list_,
                     OnCompositingEnded(this));
 }