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In order for the unit test suites to run with the iOS 13 Scene APIs:

- Create a test-specific scene delegate (similar to the test-specific
  app delegate). For now, this is a no-op delegate.

- Update the plist for unittests using the scene APIs to use this as
  the scene delegate.

- Early return from one test that uses a code path that expects
  SceneState objects to be created.

Bug: 1084483
Change-Id: Ibd0efbc72b55818647076481b361b3252e847520
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2241652Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783002}