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A common problem when writing integration tests for Views UI is that
the involved UI is hidden behind an interface which hides the Widget,
since it is usually an implementation detail. Specifically, it is
common to have a function like so:

  static void MyDialog::Show(...);

which internally creates a Widget, initializes it, shows it, etc, and
invokes some callbacks or notifies some controller when the Widget is
done. However, this makes it difficult for tests that expect to touch
that UI to actually get ahold of it, which leads to various hacky
approaches, like adding globals to track the last Widget created by
MyDialog and similar. However, even this hack is considerably ugly
when the UI is instead shown via something like:

  MyController::WaitUntilReadyAndThenShow(...);

because the test can rarely tell when the UI is actually being
displayed, which results in ad-hoc things like spinning the RunLoop a
set number of times.

To achieve that, this change:
1) Adds a class AnyWidgetObserver, which can be used to observe events
   that happen on any Widget instance without needing to specifically
   register for that Widget instance;
2) Adds a class AnyWidgetObserverSingleton, which is what the
   AnyWidgetObservers actually register as observers of;
3) Has Widget notify AnyWidgetObserverSingleton of various interesting
   events;
4) Adds some tests for AnyWidgetObserver.

Bug: None
Change-Id: I36c9b3c59bcae3bbe889731720e797339e5675d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2110812Reviewed-by: Dana Fried <dfried@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753332}