Add PERSISTENT_MAP usage for GpuMemoryBuffers.

ui/gfx/gpu_memory_buffer.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_GFX_GPU_MEMORY_BUFFER_H_
 #define UI_GFX_GPU_MEMORY_BUFFER_H_
 
 #include "base/memory/shared_memory.h"
 #include "build/build_config.h"
 #include "ui/gfx/gfx_export.h"
@@ skipping 42 matching lines @@
     RGBA_8888,
     RGBX_8888,
     BGRA_8888,
     YUV_420,
 
     FORMAT_LAST = YUV_420
   };
 
   // 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() {}
 
   // Maps each plane of the buffer into the client's address space so it can be
   // written to by the CPU. A pointer to plane K is stored at index K-1 of the
   // |data| array. This call may block, for instance if the GPU needs to finish
   // accessing the buffer or if CPU caches need to be synchronized. Returns
   // false on failure.
   virtual bool Map(void** data) = 0;
 
@@ skipping 14 matching lines @@
   // Returns a platform specific handle for this buffer.
   virtual GpuMemoryBufferHandle GetHandle() const = 0;
 
   // Type-checking downcast routine.
   virtual ClientBuffer AsClientBuffer() = 0;
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_GPU_MEMORY_BUFFER_H_