Add support for glMapBufferRange. Use glMapBufferRange and glMapBufferSubData.

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 148ca1b2ee6b4c1af0e6b812094f56fc0a23e096..19d23e0dac0b023d1ee677b42a21aa6df36612d7 100644
--- a/src/gpu/gl/GrGLBufferImpl.h
+++ b/src/gpu/gl/GrGLBufferImpl.h
@@ -53,6 +53,8 @@ private:
     GrGLenum     fBufferType; // GL_ARRAY_BUFFER or GL_ELEMENT_ARRAY_BUFFER
     void*        fCPUData;
     void*        fLockPtr;
+    size_t       fGLSizeInBytes;     // In certain cases we make the size of the GL buffer object
+                                     // smaller or larger than the size in fDesc.
 
     typedef SkNoncopyable INHERITED;
 };