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FtlSignalStrategy deletes underlying messaging client and other members
with a 5-second delay to allow session-terminate message to be sent
after deleting the signal strategy. The current implementation doesn't
work well at shutdown. When the sequence itself is being deleted, all
pending tasks will be deleted on an arbitrary thread. This will cause
a race condition in gRPC executor since it tries to delete unique_ptrs
bound into the closure.

Unlike CL 1692736 This CL provides a different way to solve this issue.
Currently only mobile clients use short-lived FtlSignalStrategy, so this
CL moves the delay destruction logic into ChromotingSession. Mobile
clients don't do any cleanup at shutdown, so they won't experience crash
at shutdown even if the app gets killed right after a session is
terminated.

ChromeOS enterprise IT2ME host still has a risk of session-terminate
not being sent at shutdown, but this is probably fine since the host
can't initiate a session termination (i.e. no disconnect window) AFAIK.

Bug: 979398
Change-Id: Ie12f48257e6f9c59391a7ed15224987ffdc879ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1707592
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678576}