Fix remaining non-circular gn check errors in Media Router targets

The gn check errors that remain after this change are things like
/c/b/m/router files including /c/browser files, but /c/b/m/router
depending on /c/browser would be a circular dependency.  This is
mostly due to siblings of Media Router that don't have their own
target underneath /c/browser and go directly into /c/browser:browser.

Bug: 740319
Change-Id: Iee4a9594356c1959c26ff88e154da62e3f418831
Reviewed-on: https://chromium-review.googlesource.com/758812Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Brandon Tolsch <btolsch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515009}

chrome/browser/media/router/discovery/BUILD.gn

@@ -12,6 +12,7 @@ static_library("discovery") {
     "//base:i18n",
     "//chrome/app:generated_resources",
     "//chrome/common:constants",
+    "//chrome/common/media_router:router",
     "//chrome/common/media_router/mojo:dial_utility_interfaces",
     "//components/cast_channel",
     "//components/version_info",

extensions/browser/api/cast_channel/BUILD.gn

@@ -20,6 +20,7 @@ source_set("cast_channel") {
   deps = [
     "//base",
     "//components/cast_channel",
+    "//components/cast_channel/proto:cast_channel_proto",
     "//extensions/common/api",
     "//net",
   ]