ozone/drm: Comment why it's ok to skip QueueOverlayPlane

Add a comment explaining why it's ok to silently fail to schedule an
overlay plane in GbmPixmap::ScheduleOverlayPlane when fb is 0.

Bug: 779356
Test: It's a comment
Change-Id: Id68fe9464b1b18023d16176005c8fb5af62e8c6f
Reviewed-on: https://chromium-review.googlesource.com/1057867Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558385}

ui/ozone/platform/drm/gpu/gbm_buffer.cc

@@ -375,7 +375,9 @@ bool GbmPixmap::ScheduleOverlayPlane(gfx::AcceleratedWidget widget,
                                      bool enable_blend,
                                      gfx::GpuFence* gpu_fence) {
   DCHECK(buffer_->GetFlags() & GBM_BO_USE_SCANOUT);
-
+  // |framebuffer_id| might be 0 if AddFramebuffer2 failed, in that case we
+  // already logged the error in GbmBuffer ctor. We avoid logging the error
+  // here since this method might be called every pageflip.
   if (buffer_->GetFramebufferId()) {
     surface_manager_->GetSurface(widget)->QueueOverlayPlane(
         OverlayPlane(buffer_, plane_z_order, plane_transform, display_bounds,