[Chromecast] Allow targets in //chromecast to depend on //media/mojo/clients.

Internal Cast code requires that clients in remote processes access the
mojo:media service. Allow targets in the //chromecast/ subdirectory to
depend on client code. A pattern-match is used to obscure the structure
of internal directories.

BUG=

Review-Url: https://codereview.chromium.org/2353573002
Cr-Commit-Position: refs/heads/master@{#419523}

media/mojo/clients/BUILD.gn

@@ -5,6 +5,7 @@
 # Implementations of media C++ interfaces using corresponding mojo services.
 source_set("clients") {
   visibility = [
+    "//chromecast/*",
     "//content/renderer",
 
     # TODO(xhwang): Only allow //media/mojo:media_mojo_unittests