Clean up public interface of AttachmentBrokerUnprivileged.

ipc/attachment_broker_unprivileged.cc

Index: ipc/attachment_broker_unprivileged.cc
diff --git a/ipc/attachment_broker_unprivileged.cc b/ipc/attachment_broker_unprivileged.cc
index 9286a8900574b84b7022d64d053b1f28c6c30b69..a735bf8a337d91bbe30ef53ac530a4eb4721ae28 100644
--- a/ipc/attachment_broker_unprivileged.cc
+++ b/ipc/attachment_broker_unprivileged.cc
@@ -4,6 +4,7 @@
 
 #include "ipc/attachment_broker_unprivileged.h"
 
+#include "base/lazy_instance.h"
 #include "base/metrics/histogram_macros.h"
 #include "build/build_config.h"
 #include "ipc/ipc_channel.h"
@@ -19,18 +20,15 @@
 
 namespace IPC {
 
-AttachmentBrokerUnprivileged::AttachmentBrokerUnprivileged()
-    : sender_(nullptr) {
-  IPC::AttachmentBroker::SetGlobal(this);
-}
-
-AttachmentBrokerUnprivileged::~AttachmentBrokerUnprivileged() {
-  IPC::AttachmentBroker::SetGlobal(nullptr);
-}
+namespace {
 
-// static
-scoped_ptr<AttachmentBrokerUnprivileged>
-AttachmentBrokerUnprivileged::CreateBroker() {
+// On platforms that support attachment brokering, returns a new instance of
+// a platform-specific attachment broker. Otherwise returns |nullptr|.
+// The caller takes ownership of the newly created instance, and is
+// responsible for ensuring that the attachment broker lives longer than
+// every IPC::Channel. The new instance automatically registers itself as the
+// global attachment broker.
+scoped_ptr<AttachmentBrokerUnprivileged> CreateBroker() {
 #if defined(OS_WIN)
   return scoped_ptr<AttachmentBrokerUnprivileged>(
       new IPC::AttachmentBrokerUnprivilegedWin);
@@ -42,6 +40,41 @@ AttachmentBrokerUnprivileged::CreateBroker() {
 #endif
 }
 
+// This class is wrapped in a LazyInstance to ensure that its constructor is
+// only called once. The constructor creates an attachment broker and sets it as
+// the global broker.
+class AttachmentBrokerMakeOnce {
+ public:
+  AttachmentBrokerMakeOnce() {
+    // Single process tests can cause an attachment broker to already exist.
+    if (AttachmentBroker::GetGlobal())
+      return;
+    attachment_broker_ = CreateBroker();
+  }
+
+ private:
+  scoped_ptr<IPC::AttachmentBrokerUnprivileged> attachment_broker_;
+};
+
+base::LazyInstance<AttachmentBrokerMakeOnce>::Leaky
+    g_attachment_broker_make_once = LAZY_INSTANCE_INITIALIZER;
+
+}  // namespace
+
+AttachmentBrokerUnprivileged::AttachmentBrokerUnprivileged()
+    : sender_(nullptr) {
+  IPC::AttachmentBroker::SetGlobal(this);
+}
+
+AttachmentBrokerUnprivileged::~AttachmentBrokerUnprivileged() {
+  IPC::AttachmentBroker::SetGlobal(nullptr);
+}
+
+// static
+void AttachmentBrokerUnprivileged::CreateBrokerIfNeeded() {
+  g_attachment_broker_make_once.Get();
+}
+
 void AttachmentBrokerUnprivileged::DesignateBrokerCommunicationChannel(
     Endpoint* endpoint) {
   DCHECK(endpoint);
@@ -50,6 +83,10 @@ void AttachmentBrokerUnprivileged::DesignateBrokerCommunicationChannel(
   endpoint->SetAttachmentBrokerEndpoint(true);
 }
 
+bool AttachmentBrokerUnprivileged::IsPrivilegedBroker() {
+  return false;
+}
+
 void AttachmentBrokerUnprivileged::LogError(UMAError error) {
   UMA_HISTOGRAM_ENUMERATION(
       "IPC.AttachmentBrokerUnprivileged.BrokerAttachmentError", error,