[Media Router] Update route-details styling.

Per conversation with UX, this change:
- Removes the unused (and incorrect) background color for the media-router-container.
- Updates the margins of the non-custom controller details.
- Updates the sizing and styling of the stop casting button in the route-details view.

Final changes to styling/redlines per offline conversation after mocks came out.


Review URL: https://codereview.chromium.org/1507873003

Cr-Commit-Position: refs/heads/master@{#364389}
3 files changed