cc: Do not cleanup tiles with raster tasks.

cc/resources/prioritized_tile_set.cc

Index: cc/resources/prioritized_tile_set.cc
diff --git a/cc/resources/prioritized_tile_set.cc b/cc/resources/prioritized_tile_set.cc
index b7b3b5a741eee424707201abc7453cfbb776d115..82c7f16facdd4567e472e7d262f6f04ded652a01 100644
--- a/cc/resources/prioritized_tile_set.cc
+++ b/cc/resources/prioritized_tile_set.cc
@@ -81,6 +81,14 @@ void PrioritizedTileSet::Clear() {
   }
 }
 
+bool PrioritizedTileSet::IsEmpty() {
+  for (int bin = 0; bin < NUM_BINS; ++bin)
+    if (!tiles_[bin].empty())
+      return false;
+
+  return true;
+}
+
 void PrioritizedTileSet::SortBinIfNeeded(ManagedTileBin bin) {
   if (!bin_sorted_[bin]) {
     SortBinTiles(bin, &tiles_[bin]);