zero-copy: Clarify to allocate/destroy GpuMemoryBuffer on any thread and use it on the main thread o

content/common/gpu/gpu_memory_buffer_factory.h

Index: content/common/gpu/gpu_memory_buffer_factory.h
diff --git a/content/common/gpu/gpu_memory_buffer_factory.h b/content/common/gpu/gpu_memory_buffer_factory.h
index 467944b63faff04f5a9c2be3cfb0923bddfe86ab..5eb0dfce1a93775074ca79fb72d16d38acee522a 100644
--- a/content/common/gpu/gpu_memory_buffer_factory.h
+++ b/content/common/gpu/gpu_memory_buffer_factory.h
@@ -31,7 +31,6 @@ class CONTENT_EXPORT GpuMemoryBufferFactory {
     gfx::GpuMemoryBuffer::Usage usage;
   };
 
-  GpuMemoryBufferFactory() {}
   virtual ~GpuMemoryBufferFactory() {}
 
   // Gets system supported GPU memory buffer factory types. Preferred type at
@@ -47,7 +46,7 @@ class CONTENT_EXPORT GpuMemoryBufferFactory {
       std::vector<Configuration>* configurations) = 0;
 
   // Creates a new GPU memory buffer instance. A valid handle is returned on
-  // success.
+  // success. It can be called on any thread.
   virtual gfx::GpuMemoryBufferHandle CreateGpuMemoryBuffer(
       gfx::GpuMemoryBufferId id,
       const gfx::Size& size,
@@ -57,12 +56,16 @@ class CONTENT_EXPORT GpuMemoryBufferFactory {
       gfx::PluginWindowHandle surface_handle) = 0;
 
   // Destroys GPU memory buffer identified by |id|.
+  // It can be called on any thread.
   virtual void DestroyGpuMemoryBuffer(gfx::GpuMemoryBufferId id,
                                       int client_id) = 0;
 
   // Type-checking downcast routine.
   virtual gpu::ImageFactory* AsImageFactory() = 0;
 
+ protected:
+  GpuMemoryBufferFactory() {}
+
  private:
   DISALLOW_COPY_AND_ASSIGN(GpuMemoryBufferFactory);
 };