[Media Router] Reland of "Media Router GYP/GN cleanup".

Removed extra 'dependency' on target 'media_router_test_support'.

Also removed the ':' on some local dependencies.

Original patch: https://codereview.chromium.org/1136623003/
Original patch description:

- Refactor GYP and GN files to share source lists.
- Extract a proper test_support target.
- Fix upstream files with new target structure.

BUG=461815, 464199, 464205
TEST=Linux trybots. It's not clear to be exactly which bot, since
original patch failed after getting through trybots/CQ.

Review URL: https://codereview.chromium.org/1122393005

Cr-Original-Commit-Position: refs/heads/master@{#330204}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: b6b09239feb034d546976028a15c75404579d0bb

config/BUILD.gn

@@ -248,6 +248,9 @@ config("feature_flags") {
   } else {
     defines += [ "CHROMIUM_BUILD" ]
   }
+  if (enable_media_router) {
+    defines += [ "ENABLE_MEDIA_ROUTER=1" ]
+  }
 }
 
 # Debug/release ----------------------------------------------------------------