[A11yPerformance] Better AXObject ToString() in uninitialized cases

This keeps the protections against confusing segfaults from
crrev.com/c/6282805, but does not prohibit ToString() before Init()
completes. Without this, errors such those in crrev.com/c/6543620
are masked, because we get an error that ToString() was used
too early, rather than the actual error.

Bug: none
Change-Id: I40bb7a01723ad8a0bf8b9b9310577bca176024a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6557122
Reviewed-by: Lucas Radaelli <lucasradaelli@google.com>
Commit-Queue: Lucas Radaelli <lucasradaelli@google.com>
Auto-Submit: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1461598}