Adds conversion for USHORT2N decltype

BUG=angle:643

Change-Id: I90a5e5c91351b9dd43a841cc5612f3f3656dc5b1
Reviewed-on: https://chromium-review.googlesource.com/198233
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>

src/libGLESv2/renderer/d3d9/formatutils9.cpp

@@ -469,6 +469,7 @@ GLenum GetDeclTypeComponentType(D3DDECLTYPE declType)
       case D3DDECLTYPE_SHORT4N:  return GL_SIGNED_NORMALIZED;
       case D3DDECLTYPE_USHORT4N: return GL_UNSIGNED_NORMALIZED;
       case D3DDECLTYPE_SHORT2N:  return GL_SIGNED_NORMALIZED;
+      case D3DDECLTYPE_USHORT2N: return GL_UNSIGNED_NORMALIZED;
       default: UNREACHABLE();    return GL_NONE;
     }
 }