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This is a preparation of auto complete handling.
This change adds a new interface AXTreeSourceArc::Hook, which hooks
serialization process of AXTreeSourceArc and can modify the default
output of the serialization for special Android widgets.

See go/arc-autocomplete-a11y for details.

AX-Relnotes: n/a.
Bug: b:150827488
Test: unit tests
Change-Id: Ifa9a6cf4801e97ebca58ebe9636be66910c3b3a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2569199
Commit-Queue: Hiroki Sato <hirokisato@chromium.org>
Reviewed-by: Sara Kato <sarakato@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836979}