cc: Add a tile manager check to try and diagnose renderer hangs.

This patch adds a check to ensure that all of the required tiles are
in the now bin.

R=ericrk@chromium.org, sunnyps@chromium.org
BUG=622080
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2566603004
Cr-Commit-Position: refs/heads/master@{#437952}

cc/tiles/tile_manager.cc

@@ -1071,7 +1071,10 @@ bool TileManager::AreRequiredTilesReadyToDraw(
   // have to iterate the queue to check whether the required tiles are ready to
   // draw.
   for (; !raster_priority_queue->IsEmpty(); raster_priority_queue->Pop()) {
-    if (!raster_priority_queue->Top().tile()->draw_info().IsReadyToDraw())
+    const auto& prioritized_tile = raster_priority_queue->Top();
+    // TODO(vmpstr): Check to debug crbug.com/622080. Remove when fixed.
+    CHECK_EQ(prioritized_tile.priority().priority_bin, TilePriority::NOW);
+    if (!prioritized_tile.tile()->draw_info().IsReadyToDraw())
       return false;
   }