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This change implements support in the AccessibilityBridge to deal with
multiple semantic trees, each one representing an iframe.

The trees are combined before they are sent to Fuchsia into a single
semantic tree.

Test: AccessibilityBridgeTest.*
Bug: fuchsia:63612,fuchsia:63610,fuchsia64329
Change-Id: I174ec9db254647edb33ada5b31eb24595debe031
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2556462Reviewed-by: Sharon Yang <yangsharon@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Lucas Radaelli <lucasradaelli@google.com>
Cr-Commit-Position: refs/heads/master@{#831997}