Use WaitableEvents to wake up sync IPC waiting

mojo/public/cpp/bindings/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 interfaces_bindings_gen_dir = "$root_gen_dir/mojo/public/interfaces/bindings"
 
 component("bindings") {
   sources = [
     # Normally, targets should depend on the source_sets generated by mojom
     # targets. However, the generated source_sets use portions of the bindings
@@ skipping 84 matching lines @@
     "lib/pipe_control_message_proxy.cc",
     "lib/scoped_interface_endpoint_handle.cc",
     "lib/serialization.h",
     "lib/serialization_context.cc",
     "lib/serialization_context.h",
     "lib/serialization_forward.h",
     "lib/serialization_util.h",
     "lib/string_serialization.h",
     "lib/string_traits_string16.cc",
     "lib/sync_call_restrictions.cc",
+    "lib/sync_event_watcher.cc",
     "lib/sync_handle_registry.cc",
     "lib/sync_handle_watcher.cc",
     "lib/template_util.h",
     "lib/union_accessor.h",
     "lib/validate_params.h",
     "lib/validation_context.cc",
     "lib/validation_context.h",
     "lib/validation_errors.cc",
     "lib/validation_errors.h",
     "lib/validation_util.cc",
@@ skipping 15 matching lines @@
     "string_data_view.h",
     "string_traits.h",
     "string_traits_stl.h",
     "string_traits_string16.h",
     "string_traits_string_piece.h",
     "strong_associated_binding.h",
     "strong_binding.h",
     "strong_binding_set.h",
     "struct_ptr.h",
     "sync_call_restrictions.h",
+    "sync_event_watcher.h",
     "sync_handle_registry.h",
     "sync_handle_watcher.h",
     "thread_safe_interface_ptr.h",
     "type_converter.h",
     "union_traits.h",
     "unique_ptr_impl_ref_traits.h",
   ]
 
   public_deps = [
     ":struct_traits",
@@ skipping 32 matching lines @@
     ]
 
     public_deps = [
       ":bindings",
       "//third_party/WebKit/Source/wtf",
     ]
 
     public_configs = [ "//third_party/WebKit/Source:config" ]
   }
 }