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/1175508Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583447}

base/BUILD.gn

@@ -2344,6 +2344,7 @@ test("base_unittests") {
     "metrics/statistics_recorder_unittest.cc",
     "native_library_unittest.cc",
     "no_destructor_unittest.cc",
+    "observer_list_threadsafe_unittest.cc",
     "observer_list_unittest.cc",
     "optional_unittest.cc",
     "os_compat_android_unittest.cc",