cc: Add some debug checks.

Add some DCHECK-only code to validate that a CompositorFrame is always
submitted only between calls to WillBeginImplFrame() and
DidFinishImplFrame() (i.e. when a valid BeginFrameArgs is being tracked
by the LayerTreeHostImpl).

Change-Id: I9f344135163d8fabf15c7b670a4a80eb25079e18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894912
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713722}

cc/trees/layer_tree_host_impl.cc

@@ -2074,6 +2074,10 @@ void LayerTreeHostImpl::OnDraw(const gfx::Transform& transform,
   // external viewport to be set otherwise.
   DCHECK(active_tree_->internal_device_viewport().origin().IsOrigin());
 
+#if DCHECK_IS_ON()
+  base::AutoReset<bool> reset_sync_draw(&doing_sync_draw_, true);
+#endif
+
   if (skip_draw) {
     client_->OnDrawForLayerTreeFrameSink(resourceless_software_draw_, true);
     return;
@@ -2297,6 +2301,21 @@ bool LayerTreeHostImpl::DrawLayers(FrameData* frame) {
       std::move(compositor_frame),
       /*hit_test_data_changed=*/false, debug_state_.show_hit_test_borders);
 
+#if DCHECK_IS_ON()
+  if (!doing_sync_draw_) {
+    // The throughput computation (in |FrameSequenceTracker|) depends on the
+    // compositor-frame submission to happen while a BeginFrameArgs is 'active'
+    // (i.e. between calls to WillBeginImplFrame() and DidFinishImplFrame()).
+    // Verify that this is the case.
+    // No begin-frame is available when doing sync draws, so avoid doing this
+    // check in that case.
+    const auto& bfargs = current_begin_frame_tracker_.Current();
+    const auto& ack = compositor_frame.metadata.begin_frame_ack;
+    DCHECK_EQ(bfargs.source_id, ack.source_id);
+    DCHECK_EQ(bfargs.sequence_number, ack.sequence_number);
+  }
+#endif
+
   frame_trackers_.NotifySubmitFrame(
       compositor_frame.metadata.frame_token, frame->has_missing_content,
       frame->begin_frame_ack, frame->origin_begin_main_frame_args);

cc/trees/layer_tree_host_impl.h

@@ -1293,6 +1293,11 @@ class CC_EXPORT LayerTreeHostImpl : public InputHandler,
   // not we are in that state.
   bool pending_tree_fully_painted_ = false;
 
+#if DCHECK_IS_ON()
+  // Use to track when doing a synchronous draw.
+  bool doing_sync_draw_ = false;
+#endif
+
   // Provides support for PaintWorklets which depend on input properties that
   // are being animated by the compositor (aka 'animated' PaintWorklets).
   // Responsible for storing animated custom property values and for