Mirroring Service: Create network context as requested.

This CL adds implementation to create a network context through Network
Service for the Mirroring Service.

Bug: 734672
Change-Id: I26422b4f4f5c69bd8398dc3ae3d5ee501c996eea
Reviewed-on: https://chromium-review.googlesource.com/1139218Reviewed-by: Yuri Wiitala <miu@chromium.org>
Reviewed-by: oysteine <oysteine@chromium.org>
Commit-Queue: Xiangjun Zhang <xjz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577635}

base/trace_event/memory_infra_background_whitelist.cc

@@ -189,6 +189,17 @@ const char* const kAllocatorDumpNameWhitelist[] = {
     "net/url_request_context/main_media/0x?/http_cache/memory_backend",
     "net/url_request_context/main_media/0x?/http_cache/simple_backend",
     "net/url_request_context/main_media/0x?/http_network_session",
+    "net/url_request_context/mirroring",
+    "net/url_request_context/mirroring/0x?",
+    "net/url_request_context/mirroring/0x?/cookie_monster",
+    "net/url_request_context/mirroring/0x?/cookie_monster/cookies",
+    "net/url_request_context/mirroring/0x?/cookie_monster/tasks_pending_global",
+    "net/url_request_context/mirroring/0x?/cookie_monster/"
+    "tasks_pending_for_key",
+    "net/url_request_context/mirroring/0x?/http_cache",
+    "net/url_request_context/mirroring/0x?/http_cache/memory_backend",
+    "net/url_request_context/mirroring/0x?/http_cache/simple_backend",
+    "net/url_request_context/mirroring/0x?/http_network_session",
     "net/url_request_context/proxy",
     "net/url_request_context/proxy/0x?",
     "net/url_request_context/proxy/0x?/cookie_monster",

chrome/browser/media/cast_mirroring_service_host.cc

@@ -9,8 +9,10 @@
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
 #include "base/task_scheduler/post_task.h"
+#include "chrome/browser/net/default_network_context_params.h"
 #include "components/mirroring/browser/single_client_video_capture_host.h"
 #include "content/public/browser/browser_thread.h"
+#include "content/public/browser/network_service_instance.h"
 #include "content/public/browser/video_capture_device_launcher.h"
 #include "mojo/public/cpp/bindings/strong_binding.h"
 
@@ -86,7 +88,11 @@ void CastMirroringServiceHost::GetVideoCaptureHost(
 
 void CastMirroringServiceHost::GetNetworkContext(
     network::mojom::NetworkContextRequest request) {
-  // TODO(xjz): Implemenation will be added in a later CL.
+  network::mojom::NetworkContextParamsPtr network_context_params =
+      CreateDefaultNetworkContextParams();
+  network_context_params->context_name = "mirroring";
+  content::GetNetworkService()->CreateNetworkContext(
+      std::move(request), std::move(network_context_params));
 }
 
 void CastMirroringServiceHost::CreateAudioStream(