cc: Don't activate rasterize on demand when we have 0 memory.

cc/resources/tile_priority.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_RESOURCES_TILE_PRIORITY_H_
 #define CC_RESOURCES_TILE_PRIORITY_H_
 
 #include <algorithm>
 #include <limits>
 #include <string>
@@ skipping 92 matching lines @@
 
   TileResolution resolution;
   bool required_for_activation;
   PriorityBin priority_bin;
   float distance_to_visible;
 };
 
 std::string TilePriorityBinToString(TilePriority::PriorityBin bin);
 
 enum TileMemoryLimitPolicy {
-  // Nothing.
+  // Nothing. This mode is used when visible is set to false.
   ALLOW_NOTHING = 0,
 
   // You might be made visible, but you're not being interacted with.
   ALLOW_ABSOLUTE_MINIMUM = 1,  // Tall.
 
   // You're being interacted with, but we're low on memory.
   ALLOW_PREPAINT_ONLY = 2,  // Grande.
 
   // You're the only thing in town. Go crazy.
   ALLOW_ANYTHING = 3,  // Venti.
-
   NUM_TILE_MEMORY_LIMIT_POLICIES = 4,
 
   // NOTE: Be sure to update TreePriorityAsValue and kBinPolicyMap when adding
   // or reordering fields.
 };
 std::string TileMemoryLimitPolicyToString(TileMemoryLimitPolicy policy);
 
 enum TreePriority {
   SAME_PRIORITY_FOR_BOTH_TREES,
   SMOOTHNESS_TAKES_PRIORITY,
@@ skipping 30 matching lines @@
   bool operator!=(const GlobalStateThatImpactsTilePriority& other) const {
     return !(*this == other);
   }
 
   void AsValueInto(base::debug::TracedValue* dict) const;
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_TILE_PRIORITY_H_