cc: Make tiles that are not required by trees NEVER_BIN

cc/resources/tile_priority.h

Index: cc/resources/tile_priority.h
diff --git a/cc/resources/tile_priority.h b/cc/resources/tile_priority.h
index 33d69f0c925dd6bd122729903a16186db86b304b..9a951561b6134e47a49cf27fab289c57bf44e30c 100644
--- a/cc/resources/tile_priority.h
+++ b/cc/resources/tile_priority.h
@@ -45,7 +45,8 @@ struct CC_EXPORT TilePriority {
       : resolution(NON_IDEAL_RESOLUTION),
         required_for_activation(false),
         time_to_visible_in_seconds(std::numeric_limits<float>::infinity()),
-        distance_to_visible_in_pixels(std::numeric_limits<float>::infinity()) {}
+        distance_to_visible_in_pixels(std::numeric_limits<float>::infinity()),
+        required_by_tree(false) {}
 
   TilePriority(TileResolution resolution,
                float time_to_visible_in_seconds,
@@ -53,7 +54,8 @@ struct CC_EXPORT TilePriority {
       : resolution(resolution),
         required_for_activation(false),
         time_to_visible_in_seconds(time_to_visible_in_seconds),
-        distance_to_visible_in_pixels(distance_to_visible_in_pixels) {}
+        distance_to_visible_in_pixels(distance_to_visible_in_pixels),
+        required_by_tree(true) {}
 
   TilePriority(const TilePriority& active, const TilePriority& pending) {
     if (active.resolution == HIGH_RESOLUTION ||
@@ -74,6 +76,9 @@ struct CC_EXPORT TilePriority {
     distance_to_visible_in_pixels =
       std::min(active.distance_to_visible_in_pixels,
                pending.distance_to_visible_in_pixels);
+
+    required_by_tree =
+        active.required_by_tree || pending.required_by_tree;
   }
   void set_current_screen_quad(const gfx::QuadF& q) { current_screen_quad = q; }
 
@@ -119,6 +124,7 @@ struct CC_EXPORT TilePriority {
   bool required_for_activation;
   float time_to_visible_in_seconds;
   float distance_to_visible_in_pixels;
+  bool required_by_tree;
 
  private:
   gfx::QuadF current_screen_quad;