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Uses automation 'find' function and also only searches
changed subtrees after the initial search. This seems to improve
performance ~4x of the back button and menu, noticeable when
autoscan is on and set to high scanning speeds.

4x improvement measured on a webpage with a lot of nodes using
devtools 'performance' tab:
Before: Just under 4000ms on the back button moveForward call on
en.wikipedia.org/wiki/cat
After: Just under 1000ms on the back button moveForward call on
en.wikipedia.org/wiki/cat.

AX-Relnotes: N/A
Bug: 1108619
Change-Id: I9bd4bb1b92cde455949558c621700a6b900f2579
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2324118Reviewed-by: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792459}