Make tree id sequence in LayerTreeHost thread-safe.

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 5a3ca045a91f03f85703438a21c6eb54e49b8bde..aa057e0ad93fa61aca546c56f5452d392370c4bf 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -121,9 +121,6 @@ class CC_EXPORT LayerTreeHost {
 
   void SetLayerTreeHostClientReady();
 
-  // Returns true if any LayerTreeHost is alive.
-  static bool AnyLayerTreeHostInstanceExists();
-
   void set_needs_filter_context() { needs_filter_context_ = true; }
   bool needs_offscreen_context() const {
     return needs_filter_context_;