Avoids race during VideoStreamEncoder unittest teardown
Avoids race during VideoStreamEncoder unittest teardown

The ScopedFakeClock contains a lock. Due to declaration order, this is
the first member of VideoStreamEncoderTest to be destroyed. However,
there are cyclic tasks that may still be running at that time, and they
may try to read the time, so if we're unlucky they may trigger a use
after free condition.

This only affects test and is simply solved by moving the declaration
to before the classes that uses it.

Bug: webrtc:10929
Change-Id: I998d5ced877f355e4a45ee5cf75b2eb75faa6113
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150795
Reviewed-by: Ilya Nikolaevskiy <76788ac3efd229e9d156135d9ceb7fef65c45da4@webrtc.org>
Commit-Queue: Erik Språng <3ab8c90c99d6dd7822e0fad803ab0211dd287ae5@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29008}

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