[CrOS Cellular] Fix two issues with FakeOtaActivator.

(1) Make the destructor virtual and override base class destructors.
(2) Remove extraneous #endif from the .cc file.

These issues were caught by vkuzkokov@ on
https://chromium-review.googlesource.com/c/chromium/src/+/1604213

Change-Id: I1f82c04139c57880a46acfe904a528853f589fe2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628938
Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Vladislav Kuzkokov <vkuzkokov@chromium.org>
Reviewed-by: Vladislav Kuzkokov <vkuzkokov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663154}

chromeos/services/cellular_setup/fake_ota_activator.cc

@@ -18,5 +18,3 @@ FakeOtaActivator::~FakeOtaActivator() = default;
 }  // namespace cellular_setup
 
 }  // namespace chromeos
-
-#endif  // CHROMEOS_SERVICES_CELLULAR_SETUP_FAKE_OTA_ACTIVATOR_H_

chromeos/services/cellular_setup/fake_ota_activator.h

@@ -18,7 +18,7 @@ namespace cellular_setup {
 class FakeOtaActivator : public OtaActivator, public FakeCarrierPortalHandler {
  public:
   explicit FakeOtaActivator(base::OnceClosure on_finished_callback);
-  ~FakeOtaActivator();
+  ~FakeOtaActivator() override;
 
   using OtaActivator::InvokeOnFinishedCallback;