From the EXT_buffer_storage overview:

    The GL_EXT_texture_storage extension added immutable storage for
    texture objects (and was subsequently incorporated into OpenGL ES
    3.0). This extension further applies the concept of immutable
    storage to buffer objects.

    [T]his extension introduces the concept of persistent client
    mappings of buffer objects, which allow clients to retain pointers
    to a buffer's data store returned as the result of a mapping, and to
    issue drawing commands while those mappings are in place.

The initial implementation of EXT_buffer_storage didn't enable this
portion of the extension, so ANGLE is generating errors while attempting
to draw with an immutable buffer mapped with the GL_MAP_PERSISTENT_BIT
flag.

This CL enables that functionality, since apps (e.g., FIFA Soccer) rely
on it.

Bug: angleproject:5473
Change-Id: Icf1c0597156044a342aac5e4d2abbc29b34f46b2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2596957Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>