implement the logic to set tile priorities based on current matrix

cc/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_TILE_PRIORITY_H_
 #define CC_TILE_PRIORITY_H_
 
+#include <limits>
+
 #include "base/memory/ref_counted.h"
 #include "cc/picture_pile.h"
 #include "ui/gfx/rect.h"
 #include "ui/gfx/size.h"
 
 namespace cc {
 
 enum WhichTree {
   // Note: these must be 0 and 1 because we index with them in various places,
   // e.g. in Tile::priority_.
   ACTIVE_TREE = 0,
   PENDING_TREE = 1
 };
 
 enum TileResolution {
   LOW_RESOLUTION = 0 ,
   HIGH_RESOLUTION = 1,
   NON_IDEAL_RESOLUTION = 2
 };
 
-struct TilePriority {
+struct CC_EXPORT TilePriority {
   TilePriority()
      : resolution(NON_IDEAL_RESOLUTION),
        time_to_visible_in_seconds(std::numeric_limits<float>::max()),
        time_to_ideal_resolution_in_seconds(std::numeric_limits<float>::max()),
        distance_to_visible_in_pixels(std::numeric_limits<float>::max()) {}
 
   TilePriority(const TilePriority& active, const TilePriority& pending) {
     if (active.resolution == HIGH_RESOLUTION ||
         pending.resolution == HIGH_RESOLUTION)
       resolution = HIGH_RESOLUTION;
@@ skipping 12 matching lines @@
     distance_to_visible_in_pixels =
       std::min(active.distance_to_visible_in_pixels,
                pending.distance_to_visible_in_pixels);
   }
 
   float time_to_needed_in_seconds() const {
     return std::min(time_to_visible_in_seconds,
                     time_to_ideal_resolution_in_seconds);
   }
 
+  static const double kMaxTimeToVisibleInSeconds;
+
+  static int manhattanDistance(const gfx::RectF& a, const gfx::RectF& b);
+
+  // Calculate the time for the |current_bounds| to intersect with the
+  // |target_bounds| given its previous location and time delta.
+  // This function should work for both scaling and scrolling case.
+  static double TimeForBoundsToIntersect(gfx::RectF previous_bounds,
+                                         gfx::RectF current_bounds,
+                                         double time_delta,
+                                         gfx::RectF target_bounds);
+
   TileResolution resolution;
   float time_to_visible_in_seconds;
   float time_to_ideal_resolution_in_seconds;
   float distance_to_visible_in_pixels;
 };
 
 enum TileMemoryLimitPolicy {
   // Nothing.
   ALLOW_NOTHING,
 
@@ skipping 19 matching lines @@
 
   size_t memory_limit_in_bytes;
 
   // Set when scrolling.
   bool smoothness_takes_priority;
 };
 
 }  // namespace cc
 
 #endif  // CC_TILE_PRIORITY_H_