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Also migrates //headless from its own unique_ptr<> based ProtocolHandler
map to use content::ProtocolHandlerMap.

Bug: 556939
Change-Id: I4ceea7f048542da1dd7a1f4a68d6ab91f75ff813
Reviewed-on: https://chromium-review.googlesource.com/989397
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548102}