Use std::list for DelayedCallbackGroup queue.

By default std::queue is backed by std::dequeue which has a large
startup cost(1). This change switches to std::list for better
performance. This callback group is less concerned with performance
as it is with memory use, so switching to std::list as the
underlying container.

(1): https://en.cppreference.com/w/cpp/container/deque

Bug: none
Change-Id: I245ec236f14ef64fcd3173cb3e1c0080c359536d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1676673
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Chris Mumford <cmumford@google.com>
Cr-Commit-Position: refs/heads/master@{#674147}