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This CL will allow projects (like WebRTC, see [1]) to configure whether
to use XCtest or not on a target by target basis instead of doing it at
build level with the GN arg 'enable_run_ios_unittests_with_xctest'.

[1] - https://webrtc-review.googlesource.com/c/src/+/184360

Bug: webrtc:11937
Change-Id: Ia227b2f5eb268dc59cfbe5617dbfc4836e6ae372
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414169Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807647}