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This adds code to enable media router features, although the
implementation is still missing.

Notably,
- a feature flag is added to enable/disable MediaRouter code
- Media-related java, new and old, is moved into weblayer_private.media

Bug: 1057100
Change-Id: Ied1cfd60e19890166568fc3facbd012285c9fabf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2427289Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811910}