Add virtual destructor to ThrottleService

This change addresses a comment in the previous ThrottleService CL,
fixing the class definition by making the destructor virtual.

Bug: 997397
Test: Run ThrottleService unit test
Change-Id: Ied2f28781075f4a766aee5243698a93acb5a122b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1834568Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Willie Koomson <wvk@google.com>
Cr-Commit-Position: refs/heads/master@{#702049}

chrome/browser/chromeos/throttle_service.h

@@ -27,7 +27,7 @@ namespace chromeos {
 class ThrottleService {
  public:
   explicit ThrottleService(content::BrowserContext* context);
-  ~ThrottleService();
+  virtual ~ThrottleService();
 
   // Functions for testing
   void NotifyObserverStateChangedForTesting();