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This CL implements the wrapper for the POST request to the FCM server.

BUG=801985

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ibba578f7181920e24b39c633111d47b717eb7bb1
Reviewed-on: https://chromium-review.googlesource.com/911108
Commit-Queue: Tatiana Gornak <melandory@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Pavel Yatsuk <pavely@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550625}