Bug 1452278 - Part 1 - Make macOS nsOSHelperAppService::GetFromTypeAndExtension() not call OS MIME API's in content r=bzbarsky,froydnj

Add a new nsExternalHelperAppService derived class named nsOSHelperAppServiceChild to be used for the MIME service, external helper app service, and external protocol service interfaces in child processes. nsOSHelperAppServiceChild overrides some methods used to get MIME and external protocol handler information from the OS and implements these methods by remoting the calls to the parent process.

This is necessary because, on Mac, querying the OS for helper application info from sandboxed content processes is unreliable and has buggy side effects.

For now, only use the new class on Mac.

Android and unix file changes r+ by gcp.
Windows files changes r+ by bobowen.
Sync messages review r+ by nfroyd.

MozReview-Commit-ID: 63BiS6VCxfn

Differential Revision: https://phabricator.services.mozilla.com/D15620

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extra : moz-landing-system : lando