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This defaults to NSObject, but can be set to a class that implements the
XCTestObservation protocol to perform one-time setup in XCTest bundles.

BUG=None

Change-Id: I3ff7abbf928a16ce6626fc99ed8fd747902cc791
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617623Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660845}