Move NetworkContext mojom into its own separate file.

I should have done this initially, but they're now past the point where they're large enough to be in their own file.

TBR=nasko@chromium.org

Change-Id: I328e03aea5f5a43407c5adb57f265cb64f300b82
Reviewed-on: https://chromium-review.googlesource.com/1057841
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Helen Li <xunjieli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558504}

services/network/network_context.h

@@ -23,7 +23,7 @@
 #include "mojo/public/cpp/bindings/strong_binding_set.h"
 #include "services/network/cookie_manager.h"
 #include "services/network/http_cache_data_remover.h"
-#include "services/network/public/mojom/network_service.mojom.h"
+#include "services/network/public/mojom/network_context.mojom.h"
 #include "services/network/public/mojom/tcp_socket.mojom.h"
 #include "services/network/public/mojom/udp_socket.mojom.h"
 #include "services/network/public/mojom/url_loader_factory.mojom.h"

services/network/public/mojom/BUILD.gn

@@ -80,6 +80,7 @@ mojom("mojom") {
     "fetch_api.mojom",
     "http_request_headers.mojom",
     "network_change_manager.mojom",
+    "network_context.mojom",
     "network_param.mojom",
     "network_service.mojom",
     "network_service_test.mojom",