Revert "[Media Router] Remove obsolete flags/features."

This reverts commit a04d299b.

Reason for revert: [sheriff] compile errors on multiple bots. https://bugs.chromium.org/p/chromium/issues/detail?id=1236311

Original change's description:
> [Media Router] Remove obsolete flags/features.
>
> Removes the following flags/features:
> - load-media-router-component-extension
> - cast-media-route-provider
> - kCastToMeetingFromCastDialog
> - kCastFeedbackDialog
> - kDialMediaRouteProvider
>
> Bug: 1179785,1028753
> Change-Id: I1d422d629eccc7ff612d0b9c915011183d65ca15
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3063781


> Commit-Queue: mark a. foltz <mfoltz@chromium.org>
> Auto-Submit: mark a. foltz <mfoltz@chromium.org>
> Reviewed-by: Filip Gorski <fgorski@chromium.org>
> Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#908260}

Bug: 1179785,1028753
Change-Id: I4634974f8c8d416beda5bd347d34c7929bb82937
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3068730
Commit-Queue: Hayato Ito <hayato@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Hayato Ito <hayato@chromium.org>
Auto-Submit: Hayato Ito <hayato@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#908292}