Menu in StocksApp should dismiss when tapping elsewhere
Menu in StocksApp should dismiss when tapping elsewhere

When the StocksApp menu is showing, the user shouldn't be able to interact with
the rest of the app. Instead, taps outside the menu should dismiss the menu.

This CL makes that happen by adding a ModalOverlay on top of the app. We might
want to do something fancier in the future using event delegation, but this
works for now.

R=rafaelw@chromium.org

Review URL: https://codereview.chromium.org/1031093002

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