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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}