[Media Router] Use common media-router-header for MR UI.
Uses a common media-router-header for all of Media Router WebUI, which now lives in media-router-container.
Time it takes from toolbar icon pressed to UI initialized (average 10 trials):
Before changes: 1031.492ms
After changes: 952.137ms
Number of elements on lab network (before / after setting data):
Before changes: 163 / 601
After changes: 121 / 523