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This change:
1) Moves the bulk of CocoaTest to a separate class, CocoaTestHelper, that
   can be held as a member
2) Removes CocoaTest::Init, which was misnamed and mostly unused, and
   replaces it with MarkCurrentWindowsAsInitial for the one use case
   that does need it
3) Removes a lot of testing macros from the CocoaTest header since they
   are either unused or only used in a single place
4) Also creates a //chrome CocoaTestHelper analog that bootstraps the
   Cocoa app bundle, for use in tests that need Cocoa but don't inherit
   from CocoaTest (eg because they inherit from another fixture)

Bug: 39725
Change-Id: I463480fbdbe4942561269478dc2f396db8ea8fb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2107766Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752000}