cc: Move pre-decodes to background worker.

cc/tiles/tile_manager.h

Index: cc/tiles/tile_manager.h
diff --git a/cc/tiles/tile_manager.h b/cc/tiles/tile_manager.h
index 86c66c4446efabb3b165138ebc0220b0ab5dca5b..1b08ae0eac6ca583720036e0881431c51509165f 100644
--- a/cc/tiles/tile_manager.h
+++ b/cc/tiles/tile_manager.h
@@ -104,6 +104,26 @@ RasterTaskCompletionStatsAsValue(const RasterTaskCompletionStats& stats);
 // should no longer have any memory assigned to them. Tile objects are "owned"
 // by layers; they automatically register with the manager when they are
 // created, and unregister from the manager when they are deleted.
+//
+// The TileManager coordinates scheduling of prioritized raster and decode work
+// across 2 different subsystems, namely the TaskGraphRunner used primarily for
+// raster work and images which must be decoded before rasterization of a tile
+// can proceed, and the CheckerImageTracker used for images decoded
+// asynchronously from raster using the |image_worker_task_runner|. The order in
+// which work is scheduled across these systems is as follows:
+//
+// 1) RequiredForActivation/Draw Tiles: These are the highest priority tiles
+// which block scheduling of any decode work for checkered-images.
+//
+// 2) Pre-paint Tiles: These are offscreen tiles which fall within the
+// pre-raster distance. The work for these tiles continues in parallel with the
+// decode work for checkered images from visible/pre-paint tiles.
+//
+// 3) Pre-decode Tiles: These are offscreen tiles which are outside the
+// pre-raster distance but have their images pre-decoded and locked. Finishing
+// work for these tiles on the TaskGraph blocks starting decode work for
+// checker-imaged pre-decode tiles.
+
 class CC_EXPORT TileManager : CheckerImageTrackerClient {
  public:
   TileManager(TileManagerClient* client,
@@ -332,7 +352,7 @@ class CC_EXPORT TileManager : CheckerImageTrackerClient {
   scoped_refptr<TileTask> CreateTaskSetFinishedTask(
       void (TileManager::*callback)());
   PrioritizedWorkToSchedule AssignGpuMemoryToTiles();
-  void ScheduleTasks(const PrioritizedWorkToSchedule& work_to_schedule);
+  void ScheduleTasks(PrioritizedWorkToSchedule work_to_schedule);
 
   void PartitionImagesForCheckering(const PrioritizedTile& prioritized_tile,
                                     const gfx::ColorSpace& raster_color_space,
@@ -341,6 +361,7 @@ class CC_EXPORT TileManager : CheckerImageTrackerClient {
   void AddCheckeredImagesToDecodeQueue(
       const PrioritizedTile& prioritized_tile,
       const gfx::ColorSpace& raster_color_space,
+      CheckerImageTracker::DecodeType decode_type,
       CheckerImageTracker::ImageDecodeQueue* image_decode_queue);
 
   std::unique_ptr<base::trace_event::ConvertableToTraceFormat>