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..5591486ca4ab5f3647d150bc2e191df807e53c73 100644
--- a/cc/tiles/checker_image_tracker.cc
+++ b/cc/tiles/checker_image_tracker.cc
@@ -37,6 +37,11 @@ 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)
@@ -47,6 +52,19 @@ CheckerImageTracker::CheckerImageTracker(ImageController* image_controller,
 
 CheckerImageTracker::~CheckerImageTracker() = default;
 
+void CheckerImageTracker::SetCanSchedulePredecodeImages(
+    bool can_schedule_predecode_images) {
+  if (can_schedule_predecode_images_ == can_schedule_predecode_images)
+    return;
+  can_schedule_predecode_images_ = can_schedule_predecode_images;
+
+  // If scheduling for the pre-decode queue was disabled, then we will stop
+  // queueing more work once a decode currently in flight is completed. If it
+  // was enabled, then kick off the next decode task.
+  if (can_schedule_predecode_images_)
+    ScheduleNextImageDecode();
+}
+
 void CheckerImageTracker::ScheduleImageDecodeQueue(
     ImageDecodeQueue image_decode_queue) {
   TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("cc.debug"),
@@ -235,9 +253,18 @@ void CheckerImageTracker::ScheduleNextImageDecode() {
   if (outstanding_image_decode_.has_value())
     return;
 
+  // If scheduling pre-decodes was disabled and we have reached the pre-decode
+  // images in the queue, don't run them now. We will come back here when
+  // these decodes are enabled.
+  if (!image_decode_queue_.empty() &&

[vmpstr, 2017/06/12 21:45:30]

Can you break this up a bit. Like, check if empty and return early. Then the
rest of the function can assume not empty.

What's guarding the ordering here? Are we guaranteed to get Raster tasks before
PreDecode ones?

[Khushal, 2017/06/14 00:29:19]

Done.
I added a DCHECK in ScheduleImageDecodeQueue to ensure that the images are
prioritized in the correct order.

+      image_decode_queue_.front().type == DecodeType::kPreDecode &&
+      !can_schedule_predecode_images_) {
+    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