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This allows embedders to define custom helper processes. They can define
the path to those by implementing ContentClient::GetChildPath(). For now
this is macOS only as the first place we need it is for macOS system
notifications.

Also updates the used message pump for services that do not run in a
sandboxed environment to use an NSApplication loop. This is necessary
to receive activation events from system notifications.

This CL is step 1 taken from rsesek@s prototype CL: crrev.com/c/2516122

Bug: 1127306
Change-Id: Iea2760b3daaa94bd797002be494c985abe8e78e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625882
Commit-Queue: Richard Knoll <knollr@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843712}