ipc: Add a lock to guard modifications of state of the AttachmentBroker.

ipc/attachment_broker.h

Index: ipc/attachment_broker.h
diff --git a/ipc/attachment_broker.h b/ipc/attachment_broker.h
index 36489d4ee9dc8133e7184aceec9cb31e19152a9f..994068dcd18983c7e061953569e5e787efef6777 100644
--- a/ipc/attachment_broker.h
+++ b/ipc/attachment_broker.h
@@ -8,6 +8,7 @@
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/process/process_handle.h"
+#include "base/synchronization/lock.h"
 #include "ipc/brokerable_attachment.h"
 #include "ipc/ipc_export.h"
 #include "ipc/ipc_listener.h"
@@ -99,6 +100,8 @@ class IPC_EXPORT AttachmentBroker : public Listener {
   // This method is exposed for testing only.
   AttachmentVector* get_attachments() { return &attachments_; }
 
+  base::Lock* get_lock() { return &lock_; }
+
  private:
 #if defined(OS_WIN)
   FRIEND_TEST_ALL_PREFIXES(AttachmentBrokerUnprivilegedWinTest,
@@ -115,6 +118,10 @@ class IPC_EXPORT AttachmentBroker : public Listener {
   AttachmentVector attachments_;
 
   std::vector<Observer*> observers_;
+
+  // The AttachmentBroker can be accessed from any thread, so modifications to
+  // internal state must be guarded by a lock.
+  base::Lock lock_;
   DISALLOW_COPY_AND_ASSIGN(AttachmentBroker);
 };