Clarification: If the omnibox is visible, it's only necessary when casting from an iframe.

Do we currently have code to detect when the presentation request is from within an iframe?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f4ca00649033653ab41306f2d5d131bbe3495bd7

commit f4ca00649033653ab41306f2d5d131bbe3495bd7
Author: Takumi Fujimoto <takumif@chromium.org>
Date: Thu Oct 04 22:59:11 2018

[Harmony Cast Dialog] Show presentation origin in dialog header

When the presentation request is set, use its origin in the dialog
header. E.g. "Cast youtube.com" instead of "Cast".

If the user selects a non-tab source (desktop or local file), switch to
using just "Cast".

Bug:  851610 
Change-Id: I7c672e458fa82af0182e1d9ed20a7f922e3ad814
Reviewed-on: https://chromium-review.googlesource.com/c/1259618
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596902}
[modify] https://crrev.com/f4ca00649033653ab41306f2d5d131bbe3495bd7/chrome/browser/ui/views/media_router/cast_dialog_view.cc
[modify] https://crrev.com/f4ca00649033653ab41306f2d5d131bbe3495bd7/chrome/browser/ui/views/media_router/media_router_views_ui.cc
[modify] https://crrev.com/f4ca00649033653ab41306f2d5d131bbe3495bd7/chrome/browser/ui/views/media_router/media_router_views_ui.h
[modify] https://crrev.com/f4ca00649033653ab41306f2d5d131bbe3495bd7/chrome/browser/ui/views/media_router/media_router_views_ui_unittest.cc