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This CL defines a layer in between the gRPC Channel and the CmaBackend,
which is responsible for converting between internal and external
entity types, and ensuring that calls to the CmaBackend occur on the
proper thread.

Bug: b/167426091
Change-Id: I7bf46475f147a62500f155eb670b4983de4ba824
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519156
Commit-Queue: Ryan Keane <rwkeane@google.com>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834565}