cc: Move pre-decodes to background worker.

cc/tiles/checker_image_tracker.cc

Index: cc/tiles/checker_image_tracker.cc
diff --git a/cc/tiles/checker_image_tracker.cc b/cc/tiles/checker_image_tracker.cc
index e076983ad7e9f3452a042accf35c69efd2895b57..4916814469b4edf6647ec9ad5fcc1d810e91a8d9 100644
--- a/cc/tiles/checker_image_tracker.cc
+++ b/cc/tiles/checker_image_tracker.cc
@@ -37,16 +37,31 @@ std::string ToString(PaintImage::Id paint_image_id,
 
 }  // namespace
 
+CheckerImageTracker::ImageDecodeRequest::ImageDecodeRequest(
+    PaintImage paint_image,
+    DecodeType type)
+    : paint_image(std::move(paint_image)), type(type) {}
+
 CheckerImageTracker::CheckerImageTracker(ImageController* image_controller,
                                          CheckerImageTrackerClient* client,
                                          bool enable_checker_imaging)
     : image_controller_(image_controller),
       client_(client),
       enable_checker_imaging_(enable_checker_imaging),
+      decode_type_allowed_(DecodeType::kNone),
       weak_factory_(this) {}
 
 CheckerImageTracker::~CheckerImageTracker() = default;
 
+void CheckerImageTracker::SetCanScheduleDecodeType(DecodeType decode_type) {
+  if (decode_type_allowed_ == decode_type)
+    return;
+  decode_type_allowed_ = decode_type;
+
+  // This will start the next decode if applicable.
+  ScheduleNextImageDecode();
+}
+
 void CheckerImageTracker::ScheduleImageDecodeQueue(
     ImageDecodeQueue image_decode_queue) {
   TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("cc.debug"),
@@ -56,6 +71,15 @@ void CheckerImageTracker::ScheduleImageDecodeQueue(
   // be checkered.
   DCHECK(image_decode_queue.empty() || enable_checker_imaging_);
 
+#if DCHECK_IS_ON()
+  // The decodes in the queue should be prioritized correctly.
+  DecodeType type = DecodeType::kRaster;
+  for (const auto& image_request : image_decode_queue) {
+    DCHECK_GE(image_request.type, type);
+    type = image_request.type;
+  }
+#endif
+
   image_decode_queue_ = std::move(image_decode_queue);
   ScheduleNextImageDecode();
 }
@@ -235,9 +259,17 @@ void CheckerImageTracker::ScheduleNextImageDecode() {
   if (outstanding_image_decode_.has_value())
     return;
 
+  if (image_decode_queue_.empty())
+    return;
+
+  // If scheduling decodes for this priority is not allowed right now, don't
+  // schedule them. We will come back here when the allowed priority changes.
+  if (image_decode_queue_.front().type > decode_type_allowed_)
+    return;
+
   DrawImage draw_image;
   while (!image_decode_queue_.empty()) {
-    auto candidate = std::move(image_decode_queue_.front());
+    auto candidate = std::move(image_decode_queue_.front().paint_image);
     image_decode_queue_.erase(image_decode_queue_.begin());
 
     // Once an image has been decoded, it can still be present in the decode