The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0c9feff82bdd88452bb151ce650fbe781518c3f9

commit 0c9feff82bdd88452bb151ce650fbe781518c3f9
Author: Takumi Fujimoto <takumif@chromium.org>
Date: Thu Jun 07 04:07:16 2018

[Harmony Cast Dialog] Show pull-down cast mode picker

This CL adds a pull-down menu button at the "extra view" area of the
Cast dialog. The menu contains actions to cast desktop or a local file.
The button is disabled if those modes aren't available.

This CL also uses a presentation source instead of tab mirroring if the
former is available, and disables the "Cast" button instead of hiding
it when not needed.

Local file casting won't be enabled until related code is moved from
MediaRouterUI to MediaRouterUIBase in a separate CL.

Screenshot:
https://drive.google.com/open?id=1fAdhcg-PnhXS1OzZbGuuyQuiV2Iw14P2

Bug:  839044 
Change-Id: I8229435e84619e2cabbc012850d734bfbde57831
Reviewed-on: https://chromium-review.googlesource.com/1081336
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565175}
[modify] https://crrev.com/0c9feff82bdd88452bb151ce650fbe781518c3f9/chrome/app/media_router_strings.grdp
[modify] https://crrev.com/0c9feff82bdd88452bb151ce650fbe781518c3f9/chrome/browser/ui/media_router/ui_media_sink.h
[modify] https://crrev.com/0c9feff82bdd88452bb151ce650fbe781518c3f9/chrome/browser/ui/views/media_router/cast_dialog_view.cc
[modify] https://crrev.com/0c9feff82bdd88452bb151ce650fbe781518c3f9/chrome/browser/ui/views/media_router/cast_dialog_view.h
[modify] https://crrev.com/0c9feff82bdd88452bb151ce650fbe781518c3f9/chrome/browser/ui/views/media_router/cast_dialog_view_unittest.cc
[modify] https://crrev.com/0c9feff82bdd88452bb151ce650fbe781518c3f9/chrome/browser/ui/views/media_router/media_router_views_ui.cc
[modify] https://crrev.com/0c9feff82bdd88452bb151ce650fbe781518c3f9/chrome/browser/ui/views/media_router/media_router_views_ui_unittest.cc