I have a WIP to do this (with tests!), but unfortunately, it breaks our integration tests.  Some other work on those will be necessary first.

https://chromium-review.googlesource.com/c/chromium/src/+/1100488

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/685314ef910fced1b65b0d0a3f2d1cfea02bb093

commit 685314ef910fced1b65b0d0a3f2d1cfea02bb093
Author: mark a. foltz <mfoltz@chromium.org>
Date: Tue Jun 19 19:57:46 2018

[Media Router] Update presentation URL for integration tests.

This CL changes the presentation URL for Media Router integration tests to
https:, so we can implement proper scheme handling in Blink in a future patch.

This is patch #3 in a five-patch change:

Patch #1 (google3): Deprecate test: in mr.TestProvider
Patch #2 (Chromium): Roll DEPS in Chromium
Patch #3 (Chromium): This patch
Patch #4 (google3): Remove test: from mr.TestProvider
Patch #5 (Chromium): Roll DEPS again

Bug:  825927 
Change-Id: I8574be618a8005c52567750c62b1945ee9d413f7
Reviewed-on: https://chromium-review.googlesource.com/1106555
Reviewed-by: Derek Cheng <imcheng@chromium.org>
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568580}
[modify] https://crrev.com/685314ef910fced1b65b0d0a3f2d1cfea02bb093/chrome/test/media_router/resources/common.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/58c569cc445d854d910e81743112c0a3664537ba

commit 58c569cc445d854d910e81743112c0a3664537ba
Author: mark a. foltz <mfoltz@chromium.org>
Date: Fri Jun 22 20:58:20 2018

[Presentation API] Ignore unknown schemes in the PresentationRequest ctor.

A spec change requires that URLs with unknown schemes be ignored by the
PresentationRequest ctor, to improve interoperability for vendor-specific schemes.

This updates the implementation and adds tests for Blink-specific custom
schemes.

Bug:  825927 
Change-Id: I15c72e54c1c44fa1a12139b6c01e2e460d079310
Reviewed-on: https://chromium-review.googlesource.com/1100488
Reviewed-by: Brandon Tolsch <btolsch@chromium.org>
Reviewed-by: Derek Cheng <imcheng@chromium.org>
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569767}
[delete] https://crrev.com/966fc0411b2a4e5ca7feb8bfdc27368935156258/third_party/WebKit/LayoutTests/external/wpt/presentation-api/controlling-ua/PresentationRequest_error.https-expected.txt
[add] https://crrev.com/58c569cc445d854d910e81743112c0a3664537ba/third_party/WebKit/LayoutTests/presentation/presentationrequest-ctor.html
[modify] https://crrev.com/58c569cc445d854d910e81743112c0a3664537ba/third_party/blink/renderer/modules/presentation/presentation_request.cc
[modify] https://crrev.com/58c569cc445d854d910e81743112c0a3664537ba/third_party/blink/renderer/modules/presentation/presentation_request_test.cc