The VMA allocator has a handy feature where during memory
allocation we can request persistently mapped memory.
This saves IOCTL overhead for apps that update buffers
frequently.

Bug: angleproject:2162
Change-Id: I870d880033beec343efae6de06f1c5935de4c2c1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2155131Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>