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 c79f3e515f15a5c54dfaaef35ca4da8ff370caf5..1763990d0c708c7712a4acdcef418c92a69c85ed 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -134,9 +134,6 @@ class CC_EXPORT LayerTreeHost : NON_EXPORTED_BASE(public RateLimiterClient) {
 
   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_;