Part 2: Retire the support for 3-valued syntax for position.

According to this resolved spec issue:
w3c/csswg-drafts#2140,
we retire the 3-valued <position> on
1. `object-position`
2. `perspective-origin`,
3. `mask-position`
4. `circle()` and `ellipse()`
, but still keep the support for `background-position`.

Besides, I simply run this python script to generate the .ini file:
```
s = sys.argv[1] + ".ini"
with open(s, "w") as f:
    f.write('[{}]\n'.format(sys.argv[1]))
    f.write('  expected: FAIL\n')
    f.write('  bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1559276\n')
```

Differential Revision: https://phabricator.services.mozilla.com/D37126

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1559276
gecko-commit: 6ec2809da4e31720cbb59b4ba71616c1c168553e
gecko-integration-branch: autoland
gecko-reviewers: emilio

css/css-images/parsing/object-position-invalid.html

@@ -15,6 +15,7 @@
 test_invalid_value("object-position", "auto");
 test_invalid_value("object-position", "1px 2px 3px");
 test_invalid_value("object-position", "left right");
+test_invalid_value("object-position", "bottom 10%");
 test_invalid_value("object-position", "bottom 10% top 20%");
 
 // The following were supported in an earlier version of the spec.