MacViews: Fix widget activation.

Fixes NativeWidetMac::IsActive(), and gets some WidgetTestInteractive
activation tests passing, without regressing any views_unittests.

NativeWidetMac::IsActive had a bit of a hack to try to make it
synchronous. This once allowed tests designed for non-Desktop Aura
widgets to work the same for *desktop* widgets on Mac, but it was never
quite right.

For Aura, these tests are using non-Desktop Aura widgets, which are
parented off the root window and are able to ask the per-process aura
window manager for the active window. This avoids having to run in an
interactive_ui_test. However, the same is not true for desktop Aura
widgets (e.g. activation on desktop Linux needs to communicate with the
window server and has the same problems here as Mac if desktop widgets
are used).

On Mac, all widgets are desktop widgets for now. Calls to activate the
window are mostly processed synchronously (e.g. visibility and
layering). However, the system does not update the window reported by
[NSApplication keyWindow] in the same runloop iteration.

IsActive() on Mac is now just [NSWindow isKeyWindow], and
Widget::CanActivate() is honored.

To avoid making textfield views_unittests flaky,
test::WidgetTest::FakeWidgetIsActiveAlways() is added. On Mac, this
calls into the Objective-C runtime to force [NSWindow isKeyWindow] to
return `YES`.

To avoid making a subset of WidgetTests using GetWidgetShowState()
flaky, these tests (and GetWidgetShowState()) are moved to
WidgetTestInteractive, and a WidgetObserver used on Mac to ensure an
activation is processed by the OS.

Moving these previously non-interactive UI tests to interactive_ui_tests
isn't ideal. However, it means a follow-up can enable these tests for
desktop widgets on other (non-Mac) desktop platforms, which use
different codepaths and are currently missing out on some coverage.

Gets the following tests passing:
WidgetTestInteractive.CanActivateFlagIsHonored
WidgetTestInteractive.DisableViewDoesNotActivateWidget
WidgetTestInteractive.ViewFocusOnWidgetActivationChanges

BUG=402679

Review URL: https://codereview.chromium.org/840183002

Cr-Commit-Position: refs/heads/master@{#311597}

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