Note that ExceptionCode::kUnknownError is not IDB-specific

third_party/WebKit/Source/core/dom/ExceptionCode.h

Index: third_party/WebKit/Source/core/dom/ExceptionCode.h
diff --git a/third_party/WebKit/Source/core/dom/ExceptionCode.h b/third_party/WebKit/Source/core/dom/ExceptionCode.h
index 345779c4346b3c93a9769f69687f65642f902232..6e90967e61b9c57b5c3415783a8f98847b6faa7f 100644
--- a/third_party/WebKit/Source/core/dom/ExceptionCode.h
+++ b/third_party/WebKit/Source/core/dom/ExceptionCode.h
@@ -64,8 +64,11 @@ enum {
   kInvalidNodeTypeError,
   kDataCloneError,
 
-  // These are IDB-specific.
+  // The operation failed for an unknown transient reason (e.g. out of memory).
+  // Note: Rethrowed V8 exception will also have this code.
   kUnknownError,
+
+  // These are IDB-specific.
   kConstraintError,
   kDataError,
   kTransactionInactiveError,