ValidateBufferStorageEXT() has a copy/paste error where the flag
GL_MAP_PERSISTENT_BIT_EXT is enabled twice while
GL_MAP_COHERENT_BIT_EXT is not enabled at all during parameter checking,
causing ANGLE to reject valid calls to glBufferStorageEXT().

Bug: angleproject:5473
Test: BufferStorageTestES3.BufferStorageFlagsPersistentCoherentWrite
Change-Id: I05596f54d48118f609ef1c88ca222d9fcdb2dd3a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2596956
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>