performance_manager: Add an IsClosing property to PageNode

The Performance Manager needs to know when a page is closing to make
more intelligent resource management decisions. For example, this
state can be used to boost a closing page's priority to ensure a
smooth and fast user experience.

This CL introduces the necessary infrastructure by adding a new
`IsClosing` boolean property to `PageNode`, along with its accessor
methods and the `OnIsClosingChanged` observer notification. The state
will be set by browser-side instrumentation in a subsequent change.

Bug: 432275395

Change-Id: I6b14825175c5d90edb2692e657e68542e7236cc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6852763
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Auto-Submit: Alex Attar <aattar@google.com>
Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1504074}