cc: Add a TileManager check to ensure images exist before unreffing.

This patch adds a check to ensure that we have the images that we
expect to have (even if it's an empty set).

R=khushalsagar@chromium.org

Bug: 757049
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ic544760e175be2bf95d467008cb3b5a59dc32a35
Reviewed-on: https://chromium-review.googlesource.com/703505Reviewed-by: Khushal <khushalsagar@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506909}

cc/tiles/tile_manager.cc

@@ -1230,6 +1230,11 @@ void TileManager::OnRasterTaskCompleted(
 
   // Unref all the images.
   auto images_it = scheduled_draw_images_.find(tile_id);
+  // Every raster task unconditionally creates sync_decoded_images_ entry in
+  // CreateRasterTask. This is the only place it's cleared. So we should have
+  // the images_it here that doesn't point to end. This check is here to debug
+  // crbug.com/757049.
+  CHECK(images_it != scheduled_draw_images_.end());
   image_controller_.UnrefImages(images_it->second);
   scheduled_draw_images_.erase(images_it);