[A11y] Only call PrevOnLine/NextOnLine once per object

At least locally, each function is called 3 times, and the compiler
needs help to optimize things. Save the values in a temporary variable
and reuse them.

Also, at this point, we are serializing, which means the tree is frozen.
It's impossible to have detached objects when the tree is frozen.

Bug: none
Change-Id: I553ac88a26ba1292fd62f00fb2947ebd5b561bc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5287196
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Auto-Submit: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Cr-Commit-Position: refs/heads/main@{#1259446}