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This patch closes a Cast virtual connection between a client and a
session in two cases:

1. The corresponding PresentationConnection is closed by the renderer.
2. The client sends a "leave session" message.

Case #2 could be implemented by #1, but for now, they are both
supported for backwards compatibility.

Bug: 1059053
Change-Id: I4bfaf5fd4a49adc16980f9ef12b2502c308c1770
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2099606Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751157}