Improve unit testing of socket API (reland).

chrome/browser/extensions/api/socket/socket_api.cc

Index: chrome/browser/extensions/api/socket/socket_api.cc
diff --git a/chrome/browser/extensions/api/socket/socket_api.cc b/chrome/browser/extensions/api/socket/socket_api.cc
index 765035c42d49bcef6953f23f21dc14cce7f0f616..33581fa70208a7d43e7e2d4b92eb391c935bb471 100644
--- a/chrome/browser/extensions/api/socket/socket_api.cc
+++ b/chrome/browser/extensions/api/socket/socket_api.cc
@@ -42,6 +42,10 @@ SocketAsyncApiFunction::~SocketAsyncApiFunction() {
 
 bool SocketAsyncApiFunction::PrePrepare() {
   manager_ = ExtensionSystem::Get(profile())->socket_manager();
+  DCHECK(manager_) << "There is no socket manager. "
+    "If this assertion is failing during a test, then it is likely that "
+    "TestExtensionSystem is failing to provide an instance of "
+    "ApiResourceManager<Socket>.";
   return manager_ != NULL;
 }