cc: Compute the inflated rect to cover a fixed number of tiles.

cc/tile_priority.cc

Index: cc/tile_priority.cc
diff --git a/cc/tile_priority.cc b/cc/tile_priority.cc
index 128205313e8bf66dd61d90d9923846d0a8f05a27..760c091392393b682592b7141ca6ddd8f85b80fc 100644
--- a/cc/tile_priority.cc
+++ b/cc/tile_priority.cc
@@ -56,6 +56,10 @@ namespace cc {
 
 const float TilePriority::kMaxDistanceInContentSpace = 4096.0f;
 
+// At 256x256 tiles, 80 tiles cover an area of ~1280x4906 pixels.
+const int64 TilePriority::
+    kNumTilesToCoverWithInflatedViewportRectForPrioritization = 80;
+
 scoped_ptr<base::Value> WhichTreeAsValue(WhichTree tree) {
   switch (tree) {
   case ACTIVE_TREE: