a11y: Update accessible URL on tab selection changes

NSWindow.accessibilityDocument was not being updated when users switch
tabs via keyboard shortcuts (Cmd-Tab) or other selection methods. The
URL was only updated when tabs finished loading, not when switching
between already-loaded tabs.

Bug: 422947637
Change-Id: I74a0b2bea5f61ed6042bfef0f450d321bf22cbb5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6936241
Reviewed-by: Tzarial <zork@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Alexander Surkov <asurkov@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1514223}