[Sync] Refactoring of sync integration test checkers to remove boilerplate await methods.

chrome/browser/sync/test/integration/two_client_uss_sync_test.cc

Index: chrome/browser/sync/test/integration/two_client_uss_sync_test.cc
diff --git a/chrome/browser/sync/test/integration/two_client_uss_sync_test.cc b/chrome/browser/sync/test/integration/two_client_uss_sync_test.cc
index 977df4b1d367daf7d8308d05cb0729262c51381e..865eba368647c9c978434800266db9b7e62573a9 100644
--- a/chrome/browser/sync/test/integration/two_client_uss_sync_test.cc
+++ b/chrome/browser/sync/test/integration/two_client_uss_sync_test.cc
@@ -91,22 +91,14 @@ class KeyChecker : public StatusChangeChecker,
                    public TestModelTypeService::Observer {
  public:
   KeyChecker(TestModelTypeService* service, const std::string& key)
-      : service_(service), key_(key) {}
-
-  void OnApplySyncChanges() override { CheckExitCondition(); }
-
-  bool Wait() {
-    if (IsExitConditionSatisfied()) {
-      DVLOG(1) << "Wait() -> Exit before waiting: " << GetDebugMessage();
-      return true;
-    }
-
+      : service_(service), key_(key) {
     service_->AddObserver(this);
-    StartBlockingWait();
-    service_->RemoveObserver(this);
-    return !TimedOut();
   }
 
+  ~KeyChecker() override { service_->RemoveObserver(this); }
+
+  void OnApplySyncChanges() override { CheckExitCondition(); }
+
  protected:
   TestModelTypeService* const service_;
   const std::string key_;