gfx/buffer_types.h: remove comment ref to _CPU_READ_WRITE_PERSISTENT

crrev.com/c/1570813 removed BufferUsage::GPU_READ_CPU_READ_WRITE_PERSISTENT,
this CL removes the reference to it from the comment of gfx:BufferUsage.
No new code, just comment update.

Change-Id: I7de9114200a4c3ee10b24a40a8fe909af215056f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1596361
Auto-Submit: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657389}

ui/gfx/buffer_types.h

@@ -33,9 +33,7 @@ enum class BufferFormat {
 
 // The usage mode affects how a buffer can be used. Only buffers created with
 // *_CPU_READ_WRITE_* can be mapped into the client's address space and accessed
-// by the CPU. *_CPU_READ_WRITE_PERSISTENT adds the additional condition that
-// successive Map() calls (with Unmap() calls between) will return a pointer to
-// the same memory contents. SCANOUT implies GPU_READ_WRITE.
+// by the CPU. SCANOUT implies GPU_READ_WRITE.
 // *_VDA_WRITE is for cases where a video decode accellerator writes into
 // the buffers.