HTML MessagePort as mojo::MessagePipeHandle

content/common/fileapi/webblob_messages.h

Index: content/common/fileapi/webblob_messages.h
diff --git a/content/common/fileapi/webblob_messages.h b/content/common/fileapi/webblob_messages.h
index 3b22b7f588e9883626dce4866b72e73a0f3fcb53..c5bda1fc3a41aa3ee54820dd54d27ad9c86973b9 100644
--- a/content/common/fileapi/webblob_messages.h
+++ b/content/common/fileapi/webblob_messages.h
@@ -54,11 +54,18 @@ IPC_STRUCT_TRAITS_END()
 // * 'shortcut' transport the memory up to the IPC limit so the browser can use
 //   it if it's not currently full.
 // See https://bit.ly/BlobStorageRefactor
-IPC_MESSAGE_CONTROL4(BlobStorageMsg_RegisterBlob,
-                     std::string /* uuid */,
-                     std::string /* content_type */,
-                     std::string /* content_disposition */,
-                     std::vector<storage::DataElement> /* item_descriptions */)
+//
+// NOTE: This message is synchronous to ensure that the browser is aware of the
+// UUID before the UUID is passed to another process. This protects against a
+// race condition in which the browser could be asked about a UUID that doesn't
+// yet exist from its perspective. See also https://goo.gl/bfdE64.
+//
+IPC_SYNC_MESSAGE_CONTROL4_0(
+    BlobStorageMsg_RegisterBlob,
+    std::string /* uuid */,
+    std::string /* content_type */,
+    std::string /* content_disposition */,
+    std::vector<storage::DataElement> /* item_descriptions */)
 
 IPC_MESSAGE_CONTROL4(
     BlobStorageMsg_RequestMemoryItem,