Add PERSISTENT_MAP usage for GpuMemoryBuffers.

ui/gfx/gpu_memory_buffer.h

Index: ui/gfx/gpu_memory_buffer.h
diff --git a/ui/gfx/gpu_memory_buffer.h b/ui/gfx/gpu_memory_buffer.h
index 3cbf64c1826bf705cc61676ad0a9c43588af89ac..9fd51a5a52f34cac9310e176f418e7e8b6e04c3b 100644
--- a/ui/gfx/gpu_memory_buffer.h
+++ b/ui/gfx/gpu_memory_buffer.h
@@ -60,7 +60,10 @@ class GFX_EXPORT GpuMemoryBuffer {
 
   // The usage mode affects how a buffer can be used. Only buffers created with
   // MAP can be mapped into the client's address space and accessed by the CPU.
-  enum Usage { MAP, SCANOUT, USAGE_LAST = SCANOUT };
+  // PERSISTENT_MAP adds the additional condition that successive Map() calls
+  // (with Unmap() calls between) will return a pointer to the same memory
+  // contents.
+  enum Usage { MAP, PERSISTENT_MAP, SCANOUT, USAGE_LAST = SCANOUT };
 
   virtual ~GpuMemoryBuffer() {}