Use a fake clock for rtc::Thread::PostDelayedTask test
Use a fake clock for rtc::Thread::PostDelayedTask test

The test would flake using a real clock since time may pass between
calls to PostDelayedTask which would result in the tasks running
out of the expected order.

Bug: webrtc:11208, webrtc:11219
Change-Id: Ice5fe6ec4e9bf2ce89f00c6de7ed06b89dbe88cc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/162100
Reviewed-by: Yves Gerey <0554266f02d7ad5bb902ac47b1c8525c05c3857c@google.com>
Commit-Queue: Steve Anton <93f73658da16553b94c040887a439a155812e54e@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30101}

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