[DiscardableSharedMemoryManager] MemoryPressureListener on one thread

In a future CL, all MemoryPressureListeners will have to be destroyed on
the same thread where they were created.

An alternative to this change would have been to use SequenceBound to
instantiate the MemoryPressureListener, but this change seemed simpler.

Change-Id: I5c64fa3b1df875007b70bcf4581b3bc0d10f9232
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6821795
Reviewed-by: Thiabaud Engelbrecht <thiabaud@google.com>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1497596}