Use a packed enum for buffer targets.

BUG=angleproject:2169

Change-Id: I4e08973d0e16404b7b8ee2f119e29ac502e28669
Reviewed-on: https://chromium-review.googlesource.com/723865
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/renderer/null/FramebufferNULL.cpp b/src/libANGLE/renderer/null/FramebufferNULL.cpp
index bca9f8f..71d3cdc 100644
--- a/src/libANGLE/renderer/null/FramebufferNULL.cpp
+++ b/src/libANGLE/renderer/null/FramebufferNULL.cpp
@@ -119,7 +119,7 @@
                                       void *ptrOrOffset)
 {
     const gl::PixelPackState &packState = context->getGLState().getPackState();
-    gl::Buffer *packBuffer = context->getGLState().getTargetBuffer(GL_PIXEL_PACK_BUFFER);
+    gl::Buffer *packBuffer = context->getGLState().getTargetBuffer(gl::BufferBinding::PixelPack);
 
     // Get the pointer to write to from the argument or the pack buffer
     GLubyte *pixels = nullptr;