MacViews: Fix views_unittests BubbleDelegateTest.CloseReasons
MacViews: Fix views_unittests BubbleDelegateTest.CloseReasons

Test added in r360848, fails on Mac as the bubble's parent anchor is not being
shown before the bubble is expected to appear - this causes the bubble to be
treated as 'hidden' as its parent is not visible. This should actually be
expected behaviour on all platforms, so the failure is actually indicating
there's a bug on Aura (see http://crbug/590957).

Workaround this for now by showing the bubble's anchor widget first to allow
this to pass for all platforms.

Also add ScopedFakeNSWindowFocus to all tests to simulate focus on the parent,
as well as defend against test flakiness which might be caused by window
activation issues.

BUG=579380,590957

Review URL: https://codereview.chromium.org/1637383003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#379499}

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