[FreezeCast] Fix crash when pausing

In the case where a mirroring session is paused, but then the route is
immediately ended, Chrome crashes. This is because pause is handled with
a posted task, and an object referenced in the task gets deleted when
the route ends.

This change fixes this by moving the delay to
MirroringMediaControllerHostImpl, and handling the delay using a
OneShotTimer that can be cancelled upon the route ending early.

Bug: 1464794
Change-Id: I1cbe85196120b8e5b417faff942593a57e9ec064
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4845076
Reviewed-by: Mark Foltz <mfoltz@chromium.org>
Commit-Queue: Benjamin Zielinski <bzielinski@google.com>
Cr-Commit-Position: refs/heads/main@{#1194393}