Use JSON content type from CUS.

Bug: 952753
Change-Id: I7443b3e0d6ff228113ef19ef718561ddfacd0f88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1573658
Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Auto-Submit: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652171}

components/update_client/net/network_impl.cc

@@ -116,7 +116,7 @@ void NetworkFetcherImpl::PostRequest(
   simple_url_loader_->SetRetryOptions(
       kMaxRetriesOnNetworkChange,
       network::SimpleURLLoader::RETRY_ON_NETWORK_CHANGE);
-  simple_url_loader_->AttachStringForUpload(post_data, "application/xml");
+  simple_url_loader_->AttachStringForUpload(post_data, "application/json");
   simple_url_loader_->SetOnResponseStartedCallback(base::BindOnce(
       &NetworkFetcherImpl::OnResponseStartedCallback, base::Unretained(this),
       std::move(response_started_callback)));