Revert "cc: Disable checker-imaging for decode=async tag."

This reverts commit 9bbdf45e.

Reason for revert: The patch verified that the reporting is accurate, no
crashes were introduced from the added CHECKs. And we have enough data
to check the UKM logs.

Original change's description:
> cc: Disable checker-imaging for decode=async tag.
> 
> Temporarily disabling checkering of decode=async tag images to debug
> entries in ukm logs for checkerboarded images in the control group for
> checkerimaging finch study.
> 
> R=​vmpstr@chromium.org
> 
> Bug: 833666
> Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
> Change-Id: Ib9a79262599efa17fd18e9cb71d7518492e9a7ee
> Reviewed-on: https://chromium-review.googlesource.com/1014644
> Reviewed-by: vmpstr <vmpstr@chromium.org>
> Commit-Queue: Khushal <khushalsagar@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#551513}

TBR=vmpstr@chromium.org,khushalsagar@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 833666
Change-Id: I3425d7a7b4cc6d57ec26fee7ddee8827e97992dd
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Reviewed-on: https://chromium-review.googlesource.com/1029090Reviewed-by: Khushal <khushalsagar@chromium.org>
Commit-Queue: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553837}

cc/tiles/checker_image_tracker.cc

@@ -286,9 +286,8 @@ bool CheckerImageTracker::ShouldCheckerImage(const DrawImage& draw_image,
   // specify "async" then we don't checker the images. To put it different, we
   // currently only not respect |enable_checker_imaging_| if the value was
   // specified as "async" by the developer.
-  // TODO(khushalsagar): Enable this for async decode hint images. Temporarily
-  // disabling for debugging crbug.com/833666.
-  if (!enable_checker_imaging_) {
+  if (decoding_mode_hint != PaintImage::DecodingMode::kAsync &&
+      !enable_checker_imaging_) {
     return false;
   }
 

cc/trees/layer_tree_host_impl.cc

@@ -439,7 +439,6 @@ void LayerTreeHostImpl::UpdateSyncTreeAfterCommitOrImplSideInvalidation() {
   // and the updated data for the image from the main frame.
     PaintImageIdFlatSet images_to_invalidate =
         tile_manager_.TakeImagesToInvalidateOnSyncTree();
-    CHECK(images_to_invalidate.empty() || settings_.enable_checker_imaging);
     if (ukm_manager_)
       ukm_manager_->AddCheckerboardedImages(images_to_invalidate.size());