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When a conenction has path degrading timer fired, the connection will remember
that it's path degrading until forward progress is made, i.e., new ack has
been received on the degrading path. It will then spin timer to detect future
path degrading.

Flag protected by a new flag --quic_path_degrading_alarm2.

Manual merge internal change: 193990670

Bug: 835444
Change-Id: Idf487fafb10ccd61551f4a9905714ff83bd91cb9
Reviewed-on: https://chromium-review.googlesource.com/1025143Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553315}