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This is done uniformly for all clients of update_client including
the chrome/updater.

Unfortunately, the content-type is still done by changing each
network stack, since it can't be done cleanly with a "Content-Type"
header.

Bug: 1095704

Change-Id: I16a7523e16cd846075fee8d28852b8ddb7eaf265
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2248721Reviewed-by: S. Ganesh <ganesh@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779590}