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This CL makes HeartbeatSender send request via Protobuf over HTTP. This
is OK to be checked in now since unlike FTL, our backend is ready to
accept Protobuf/HTTP requests.

Bug: 1103416
Change-Id: Iefdbcb59602436e01dbf4475cab3f6db67004e2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2311121
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Auto-Submit: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791025}