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 * Instead, show it explicitly on caller side. This is necessary as the extensions system controls the app window's visibility.

Remove ActivityViewManager
 Instead of keeping activity -> widget map, use views::Widget::GetWidgetForNativeWindow

BUG=417017
TEST=ActivityManagerTest.Basic

Review URL: https://codereview.chromium.org/596623005

Cr-Commit-Position: refs/heads/master@{#296524}