[aim][popup] Remove visibility assert from setPreserveContextOnClose_

Addresses the "Uncaught Error: Assertion failed
(chrome://omnibox-popup.top-chrome)" JS error reports tracked in
https://crbug.com/558982300, which started in M152.

crrev.com/c/8071569 made `addContext_()` reset `preserveContextOnClose_`
by calling `setPreserveContextOnClose_(false)`. That method asserts
`document.visibilityState === 'visible'`.

The browser sends `AddContext` whenever the popup state is `kAim`
(`OmniboxContextMenuController::UpdateSearchboxContext()`), not based on
actual page visibility. The renderer's visibility state is updated over
a separate channel and can also be affected by occlusion, so
`AddContext` can be delivered while the document is hidden, and the
assert throws.

The browser-initiated `SetPreserveContextOnClose` call is gated the same
way, so the assert does not describe a real invariant. Remove it. Adds a
test that delivers `addContext` while the document is hidden; it fails
with "Assertion failed" without this fix.

Bug: 558982300
Change-Id: I4ff09e9b6c89340fa87728c6057bdbd18ce60b3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8466742
Auto-Submit: Aviv Kiss <avivkiss@google.com>
Reviewed-by: Paul Adedeji <pauladedeji@google.com>
Commit-Queue: Paul Adedeji <pauladedeji@google.com>
Cr-Commit-Position: refs/heads/main@{#1707088}