cc: Optimize decode scheduling for checker-images.

cc/tiles/checker_image_tracker.h

Index: cc/tiles/checker_image_tracker.h
diff --git a/cc/tiles/checker_image_tracker.h b/cc/tiles/checker_image_tracker.h
index b2a16ecf9b2d60c9e4b9cc1371c47329c9fd406e..ee99d483e5b21d4d8d0d3f410bbb00e279f1bce9 100644
--- a/cc/tiles/checker_image_tracker.h
+++ b/cc/tiles/checker_image_tracker.h
@@ -5,6 +5,7 @@
 #ifndef CC_TILES_CHECKER_IMAGE_TRACKER_H_
 #define CC_TILES_CHECKER_IMAGE_TRACKER_H_
 
+#include <queue>
 #include <unordered_map>
 #include <vector>
 
@@ -51,6 +52,8 @@ class CC_EXPORT CheckerImageTracker {
   // Returns the set of images to invalidate on the sync tree.
   const ImageIdFlatSet& TakeImagesToInvalidateOnSyncTree();
 
+  void ResetImageDecodeQueue();
+
   void DidActivateSyncTree();
 
  private:
@@ -64,6 +67,7 @@ class CC_EXPORT CheckerImageTracker {
                           WhichTree tree) const;
 
   void ScheduleImageDecodeIfNecessary(const sk_sp<const SkImage>& image);
+  void RequestNextImageDecode();

[vmpstr, 2017/03/06 20:45:25]

Comment, and if this is what I think it is, then it should be called
ScheduleNextImageDecode

[Khushal, 2017/03/07 00:31:08]

Done.

 
   ImageController* image_controller_;
   CheckerImageTrackerClient* client_;
@@ -76,12 +80,14 @@ class CC_EXPORT CheckerImageTracker {
   // A set of images which were invalidated on the current sync tree.
   ImageIdFlatSet invalidated_images_on_current_sync_tree_;
 
-  // A set of images which are currently pending decode from the image decode
-  // service.
-  // TODO(khushalsagar): This should be a queue that gets re-built each time we
-  // do a PrepareTiles? See crbug.com/689184.
+  // The set of images in the image decode queue.
   ImageIdFlatSet pending_image_decodes_;
 
+  // The queue of images pending decode. We maintain a queue to ensure that the
+  // order in which images are decoded is aligned with the priority of the tiles
+  // dependent on these images.
+  std::queue<sk_sp<const SkImage>> image_decode_request_queue_;

[vmpstr, 2017/03/06 20:45:25]

std::queue's underlying type is std::deque which is a big memory hog as we've
discovered. Since this is just sk_sp (small objects), I think we should just use
a vector. 

[Khushal, 2017/03/07 00:31:09]

Done.

+
   // A set of images which have been decoded at least once from the
   // ImageDecodeService and should not be checkered again.
   // TODO(khushalsagar): Limit the size of this set.