Improve unit testing of socket API.

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 6ef6b4bca86e32201d04bf3d0f0c1d39bb48c5ec..b9bd48f47afc791144d1b1cf651c4283b5eec70b 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;
 }