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This implements webstore_widget app window, and launches the app when the user
click on the notification shown when an unsupported printer is detected.

Note that the notification is behind switch --enable-printer-app-search

TODO: setting app icon; error handling.

BUG=477106
TEST=None

Review URL: https://codereview.chromium.org/1123373003

Cr-Commit-Position: refs/heads/master@{#330048}