Fix race condition around rtc::ScopedFakeClock.
Fix race condition around rtc::ScopedFakeClock.

We make sure the fake clock is constructed first thing,
so that all subsequent calls to GetClockForTesting() are
consistent and non-racy.

This proper scoping also allows to remove some explicit
destructions which are no longer necessary.

Bug: webrtc:11282
Change-Id: Id9263617c2e2b025b17d9bcb9cd415d651405a8a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/166043
Commit-Queue: Yves Gerey <0554266f02d7ad5bb902ac47b1c8525c05c3857c@google.com>
Reviewed-by: Steve Anton <93f73658da16553b94c040887a439a155812e54e@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30309}

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