[DevTools] Defer client-initiated navigations while CDP command dispatch is pending

This introduces ClientNavigationThrottler, which is passed a callback that can be either dispatched synchronously or deferred until the client releases the throttle handle, and uses it to defer renderer-initiated navigations until DevTools are done with the dispatch of CDP commands. This should improve CDP API ergonomics in a fairly common case of a CDP client initiating a navigation through a renderer-side CDP command, such as evaluating an element.click() or location.assign(). Since Blink stopped deferring client-initiated navigations some time ago, we used to navigate away before the CDP command returns, so a client would get error messages in response to a command that was, in fact, correctly processed.

Bug: 357977710
Change-Id: I4f28dbf2a0ce5a00c173f99fdde17a16f7352a1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6271784
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Philip Pfaffe <pfaffe@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1424328}