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This is a mechanical change. Its purpose is allowing different wire
protocol parsers and serializers to be injected in the UpdateChecker and
PingManager instances in the UpdateClient using the Configuration instance.

The current code hardcodes an instance of ProtocolHandlerXm. There is work
in progress to support JSON.

Bug: 881076
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I40cb1605f134a5d3f168a511094b2b265bc50f25
Reviewed-on: https://chromium-review.googlesource.com/c/1289510Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Minh Nguyen <mxnguyen@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601179}