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This CL implements a helper class that adapts gRPC's async unary call
paradigm (i.e. the completion queue) into Chromium's callback-based
paradigm.

Bug: 927962
Change-Id: I15c58748a0d024dc5f6953b0312e629fcf8c24b5
Reviewed-on: https://chromium-review.googlesource.com/c/1484736
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635816}