Revert "[a11y]: swap vector of AXTreeUpdate(s) in...

Revert "[a11y]: swap vector of AXTreeUpdate(s)  in RenderAccessibilityImpl::SendAccessibilitySerialization"

This reverts commit c5635731.

Reason for revert: swap not proven to be more performant than move (not enough pinpoint evidence).

Original change's description:
> [a11y]: swap vector of AXTreeUpdate(s)  in RenderAccessibilityImpl::SendAccessibilitySerialization
>
> - experimentally yields a ~2% performance improvement
> - swap should be more efficient than std::move
> 1. AXTreeUpdate does not actually define a move assignment operator
> 2. move assignment operators are not automatically generated due to the
> presence of both a copy constructor and destructor
> 3. copy elision may or may not take place
>
> Change-Id: If16e079b4a196b79c3ba8e7831728f9200c66f8c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5407055


> Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
> Commit-Queue: David Tseng <dtseng@chromium.org>
> Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1280194}

Change-Id: I343c54a0b898052a5ff356b76bd5e73bd6783502
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5420389


Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1282140}