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Improve upon previous implementation of GetPosInSet and GetSetSize
for AXNode, which computed pos_in_set and set_size values upon every
call of either function. Introduced cache for these values in AXTree,
which maps a node's id to a struct holding its pos_in_set and set_size
values. Cache is invalidated upon updates to the AXTree.

Change-Id: I70c0878b05b53b98648af1f2b76d1e1bf6e6d294
Reviewed-on: https://chromium-review.googlesource.com/c/1351782Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613316}