Set Cancel as the default button for the external protocol dialog.

This ensures that users holding down the Enter or Space key cannot
accidentally trigger an external protocol launch.

BUG=865202

Change-Id: I2cec7b3c216b80641200c97dec2517a66b4e0b24
Reviewed-on: https://chromium-review.googlesource.com/1142705
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576340}