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This CL makes FtlClient async such that it makes the RPC call
asynchronously and runs the callback after the response is received.

Bug: 927962
Change-Id: Id1399f3edc381f0acca7780aca2959a4aa1a3737
Reviewed-on: https://chromium-review.googlesource.com/c/1487960
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635877}