The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3e73bade32a01958f624ca45bed703cbeebfc906

commit 3e73bade32a01958f624ca45bed703cbeebfc906
Author: Mounir Lamouri <mlamouri@chromium.org>
Date: Tue Oct 02 20:31:26 2018

Media Router: dismiss Android cast dialogs when clicking outside of them.

Bug: 891455
Change-Id: I3fa228b3ef6946f151510c50137cd3a172d50384
Reviewed-on: https://chromium-review.googlesource.com/c/1257678
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595965}
[modify] https://crrev.com/3e73bade32a01958f624ca45bed703cbeebfc906/chrome/android/java/src/org/chromium/chrome/browser/media/router/MediaRouteChooserDialogManager.java
[modify] https://crrev.com/3e73bade32a01958f624ca45bed703cbeebfc906/chrome/android/java/src/org/chromium/chrome/browser/media/router/MediaRouteControllerDialogManager.java

mlamouri@: Can this be marked Fixed?

Unfortunately not, it seems that the change didn't solve the problem. I need to find the time to try this at home (no cast device at the office) and investigate why it doesn't actually work.