cc: Move pre-decodes to background worker.

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 335050c6b9df5ec861be30f2f6b58f852bbebf07..b8697e8c25e8b04e72d3849cd057fa62f61038ae 100644
--- a/cc/tiles/checker_image_tracker.h
+++ b/cc/tiles/checker_image_tracker.h
@@ -33,6 +33,22 @@ class CC_EXPORT CheckerImageTrackerClient {
 // sync tree until the previous tree is activated.
 class CC_EXPORT CheckerImageTracker {
  public:
+  enum class DecodeType : uint16_t {
+    // Highest priority. Used to disable scheduling any decode work.
+    kNone,

[vmpstr, 2017/06/20 18:41:01]

This is an awkward name. Just from looking at this, I wouldn't know when to say
DecodeType::kNone. How is this used? Is there a more descriptive name we can use
here?

[Khushal, 2017/06/21 23:45:28]

I just made it all internal so we wouldn't need to expose something in this enum
to let the caller disable all decodes.

+    // Priority for images on tiles being rasterized (visible or pre-paint).
+    kRaster,
+    // Lowest priority for images on tiles in pre-decode region. These are tiles
+    // which are beyond the pre-paint region, but have their images decoded.
+    kPreDecode
+  };
+
+  struct ImageDecodeRequest {
+    ImageDecodeRequest(PaintImage paint_image, DecodeType type);
+    PaintImage paint_image;
+    DecodeType type;
+  };
+
   CheckerImageTracker(ImageController* image_controller,
                       CheckerImageTrackerClient* client,
                       bool enable_checker_imaging);
@@ -42,9 +58,13 @@ class CC_EXPORT CheckerImageTracker {
   // service and it should be be skipped during raster.
   bool ShouldCheckerImage(const DrawImage& image, WhichTree tree);
 
-  using ImageDecodeQueue = std::vector<PaintImage>;
+  // Provides a prioritized queue of images to decode.
+  using ImageDecodeQueue = std::vector<ImageDecodeRequest>;
   void ScheduleImageDecodeQueue(ImageDecodeQueue image_decode_queue);
 
+  // The max decode priority type that is allowed to run.

[vmpstr, 2017/06/20 18:41:01]

Can you use the word "max" or "limit" in the function name to make it clear that
everything specified and below can run?

[Khushal, 2017/06/21 03:35:57]

Done.

+  void SetCanScheduleDecodeType(DecodeType decode_type);
+
   // Returns the set of images to invalidate on the sync tree.
   const PaintImageIdFlatSet& TakeImagesToInvalidateOnSyncTree();
 
@@ -130,6 +150,9 @@ class CC_EXPORT CheckerImageTracker {
   // dependent on these images.
   ImageDecodeQueue image_decode_queue_;
 
+  // The max decode type that is allowed to run.
+  DecodeType decode_type_allowed_;
+
   // The currently outstanding image decode that has been scheduled with the
   // decode service. There can be only one outstanding decode at a time.
   base::Optional<PaintImage> outstanding_image_decode_;