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This CL introduces a ProtobufHttpClient class for sending REST/protobuf
requests over HTTP using Chromium net libraries. This is the first step
of moving away from gRPC.

Bug: 1103416
Change-Id: Ic9b32e166c27f1098fcf24da831e1bdf836fa95a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2293422
Auto-Submit: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787938}