Adds PBO buffer targets when extension is present

BUG=angle:581

Change-Id: I125cdd995e538db4a275657770149071406a6ad1
Reviewed-on: https://chromium-review.googlesource.com/198237
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>
diff --git a/src/libGLESv2/validationES.cpp b/src/libGLESv2/validationES.cpp
index 1ba759c..82cba3d 100644
--- a/src/libGLESv2/validationES.cpp
+++ b/src/libGLESv2/validationES.cpp
@@ -108,6 +108,8 @@
 
       case GL_PIXEL_PACK_BUFFER:
       case GL_PIXEL_UNPACK_BUFFER:
+        return context->supportsPBOs();
+
       case GL_COPY_READ_BUFFER:
       case GL_COPY_WRITE_BUFFER:
       case GL_TRANSFORM_FEEDBACK_BUFFER: