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This CL tests that the views external protocol dialog behaves correctly
when it is Accepted, Canceled, and Closed. In particular, the status of
the dialog's checkbox is verified; when the dialog is Closed without
a definite user choice of Accept or Cancel, it is ensured that the
checkbox is always reported as unchecked. This is to prevent dismissing
the dialog from accidentally persisting a permanent block on external
protocols.

The dialog is not used on ChromeOS, so the test is only run on Linux,
Windows, and MacViews.

BUG=671658

Review-Url: https://codereview.chromium.org/2610793002
Cr-Commit-Position: refs/heads/master@{#441869}