Frame identifiers have changed from int64_t to string type. This is due
to https://crbug.com/1502660 which removes access to frame routing IDs
in the renderer process. New cross-process frame identifiers are 160-bit
values (32-bit child process ID + 128-bit local frame token) and most
easily represented as strings. All other frame-related expectations and
behaviors remain the same.
Marshall Greenblatt
Riku Palomäki