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 121bd0290f1bdd53c1c499f4cc1982cd769fbd6d..dc18ed4c8c17c4793124eb12079e229dc5ee673a 100644
--- a/ui/gfx/gpu_memory_buffer.h
+++ b/ui/gfx/gpu_memory_buffer.h
@@ -59,7 +59,10 @@ 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() {}