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When the SemanticTree disconnects because of an invalid update,
the corresponding web.Frame will be closed and destroyed.

Test: AccessibilityBridgeTest.Disconnect
Bug: fuchsia:53033
Change-Id: Ic209714d54276e1b13ce81467a18fbe44b3d09b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2285321
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792283}