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 (there was no leak reported because they're deleted upon shutdown)

A few minor cleanup/improvements
* Added shortcut to close athena. This doesn't update overview/split view mode.
  I'll address it in separate CL.
* Delete unnecessary athena_content_support_lib target. (I somehow missed this in review)
* Separate app_shell specific impl in extensions_delegate  (this is prep for athena+chrome)
* Remove ActivityWidget wrapper and use views::Widget instead, as it seems to be unnecessary. This is also prep for athena+chrome, where AppActivity should use NativeAppWindowViews.

BUG=None
TEST=coverted by unittests.
TBR=jamescook@chromium.org

Review URL: https://codereview.chromium.org/498023003

Cr-Commit-Position: refs/heads/master@{#291935}