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This is needed to allow CastAppRpcInstance to know when
the object has been destroyed.

Bug: b/170144761
Test: Locally on device
Change-Id: I878ea2c45c644bd74e52bf365d8af91b0fc9ac1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490177Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Shawn Gallea <sagallea@google.com>
Cr-Commit-Position: refs/heads/master@{#819646}