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This is yet more precursor work for making AshTestHelper an
AuraTestHelper subclass.  It moves into the constructor things that
are more "general-purpose" and don't rely on any state that might be
customized after construction (e.g. reading the ViewsDelegate), and
leaves SetUp() focused more on the ash-specific shell/windowing stuff.

This changes the parameters passed to the constructor vs. setup, since
ultimately some will be relevant to the Aura side and some won't.  For
now, this requires storing the |context_factory_| on the object to
pass it from the constructor to setup, but that will go away later.

This also adds a no-args SetUp() function, which just calls the main
SetUp() function.  This is a precursor to making SetUp() and TearDown()
be virtual method overrides.

A few other trivial changes:
* Explicitly resets |session_controller_client_| during TearDown(),
  since I forgot that previously.  (Should have no effect)
* Puts GetLocalStatePrefService() into the same order in the .cc file
  as the .h file.

Should be no functional effects.

Bug: none
Change-Id: Ida64c3fc0779f3bb38a032cf8d3f490104fe88ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2124934
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754707}