cc: Don't activate pending tree until all tiles are ready

cc/layer_tree_host_impl.cc

Index: cc/layer_tree_host_impl.cc
diff --git a/cc/layer_tree_host_impl.cc b/cc/layer_tree_host_impl.cc
index ac160844b11f26821d788b6d66f719419f66fcd4..103b4ffe97dde5d3a3c1271059f116f517a6c40d 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -1008,19 +1008,14 @@ void LayerTreeHostImpl::activatePendingTreeIfNeeded()
         return;
 
     int total_pending = m_tileManager->GetTilesInBinCount(NOW_BIN, PENDING_TREE);
-    int total_active = m_tileManager->GetTilesInBinCount(NOW_BIN, ACTIVE_TREE);

[nduca, 2013/01/05 04:51:49]

The more I stare at this, the more I think we should give per-layer activation a
try...

     int drawable_pending = m_tileManager->GetDrawableTilesInBinCount(NOW_BIN, PENDING_TREE);
-    int drawable_active = m_tileManager->GetDrawableTilesInBinCount(NOW_BIN, ACTIVE_TREE);
+    int total_active = m_tileManager->GetTilesInBinCount(NOW_BIN, ACTIVE_TREE);
 
-    if (total_pending && total_active) {
-        float percent_pending = drawable_pending / static_cast<float>(total_pending);
-        float percent_active = drawable_active / static_cast<float>(total_active);
-        if (percent_pending < percent_active)
-            return;
-    }
+    // It's always fine to activate to or from an empty tree.  Otherwise, only
+    // activate once all high res visible tiles are ready on the pending tree.
+    if (total_pending && total_active && total_pending != drawable_pending)
+        return;
 
-    // If there are no pending total tiles (i.e. an empty tree), we should
-    // commit immediately.  Similarly, if there are no active tree tiles.
     activatePendingTree();
 }