Split ObserverListThreadSafe tests out into their own .cc file.

Apart from some simple classes (Foo, Adder), the existing test suites
really don't share very much code. After https://crrev.com/c/1053338
they will share even less. Split out the threaded tests to their own
file to reduce complexity.

This is largely a verbatim move (preserving order), apart from
whitespace changes.

Bug: 842987
Change-Id: I555662a5516add7267abd1608f54fd13cbd7d97c
Reviewed-on: https://chromium-review.googlesource.com/1175508
Reviewed-by: Gabriel Charette <[email protected]>
Commit-Queue: Trent Apted <[email protected]>
Cr-Commit-Position: refs/heads/master@{#583447}
3 files changed