[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}