Relax restrictions on MemoryPressureListener sync callbacks

Previously, MemoryPressureObserver would deadlock if a synchronous
callback tried to delete any MemoryPressureListener. Relax this so that
non-synchronous MemoryPressureListeners can be deleted.

Also, clarify some behavior in the documentation and add some missing
consts.

Test: base_unittests --gtest_filter=MemoryPressureListenerTest*
Change-Id: I3a457acd98286c1be5c894519d3b1a78c164fbca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6805092
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Commit-Queue: David Stevens <stevensd@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1496690}