Add transfer buffer support.

src/gpu/gl/GrGLBufferImpl.h

Index: src/gpu/gl/GrGLBufferImpl.h
diff --git a/src/gpu/gl/GrGLBufferImpl.h b/src/gpu/gl/GrGLBufferImpl.h
index 15d2f48efe44ea82d2d1d48907585666cd2e34cb..ef7ce9554106ff15079757562d25d99ef02428ad 100644
--- a/src/gpu/gl/GrGLBufferImpl.h
+++ b/src/gpu/gl/GrGLBufferImpl.h
@@ -19,10 +19,20 @@ class GrGLGpu;
  */
 class GrGLBufferImpl : SkNoncopyable {
 public:
+    enum Usage {
+        kStaticDraw_Usage = 0,
+        kDynamicDraw_Usage,
+        kStreamDraw_Usage,
+        kStreamRead_Usage,
+
+        kLast_Usage = kStreamRead_Usage
+    };
+    static const int kUsageCount = kLast_Usage + 1;
+
     struct Desc {
         GrGLuint    fID;            // set to 0 to indicate buffer is CPU-backed and not a VBO.
         size_t      fSizeInBytes;
-        bool        fDynamic;
+        Usage       fUsage;
     };
 
     GrGLBufferImpl(GrGLGpu*, const Desc&, GrGLenum bufferType);