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External protocol dialogs display the origin that initiated the
external protocol request. The initiating origin helps the user
attribute the request to a particular site, so that they can decide if
they can trust that site to launch an external application. When the
initiating origin was opaque (such as from a sandboxed iframe), the
dialog would display no origin or a generic message, so the user
didn't have any information for making a trust decision. This CL
converts the initiating origin to its precursor origin (the origin
that created the initiating origin) when creating the external
protocol dialog. Displaying the precursor origin gives the user more
useful information for making a trust decision.

Bug: 1041749
Change-Id: I0b21d20e13d7d71db361746dbb18df8d980339bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2063420
Commit-Queue: Emily Stark <estark@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742492}