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Switch Access has been experiencing performance issues on moderate to
large webpages. A large portion of this is due to the inefficiencies of
its traversal of the automation tree, recalculating the same values
sometimes hundreds of times for nodes deep in the tree.

This change utilizes techniques from dynamic programming to persist
intermediate values for continued use, reducing the asymptotic worst-
case run time from exponential in the depth of the node to linear in
the number of nodes in the subtree.

AX-Relnotes: n/a.
Bug: 1109970
Change-Id: I61368292f2e731245e4f733c2883b86293d8a74c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2360340Reviewed-by: Abigail Klein <abigailbklein@google.com>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#799856}