[Views] Fix WidgetTest.GetRestoredBounds

Test was failing on Mac because ViewsTestHelperMac::TearDown()
now DCHECKs that all Widgets are closed (added in r334985). Fix
this by closing the Widget at end of test.

Also re-enable this test on Win. It's unfortunately still broken on
Linux (see bug).

Also re-write the test a bit to make it clearer.

BUG=378134, 515369

Review URL: https://codereview.chromium.org/1268643002

Cr-Commit-Position: refs/heads/master@{#341643}