Fix formAction return value for empty formaction content attribute.

Fix formAction return value for empty formaction content attribute.

According to the standard and web-platform-tests, empty attribute value should
return the document URL.
https://html.spec.whatwg.org/multipage/forms.html#dom-fs-formaction

> The formAction IDL attribute must reflect the formaction content attribute,
> except that on getting, when the content attribute is missing or its value is
> the empty string, the document's address must be returned instead.

This CL removes empty formaction tests in submit-form-attributes.html.
html/semantics/forms/attributes-common-to-form-controls/formaction*.html cover
them.

BUG=520835
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201617

[tkent, 2015-09-02 05:21:39]

tkent@chromium.org changed reviewers:
+ keishi@chromium.org

[tkent, 2015-09-02 05:21:39]

Keishi, would you review this please?

[keishi, 2015-09-02 05:34:00]

lgtm

[tkent, 2015-09-02 05:34:59]

The CQ bit was checked by tkent@chromium.org

[commit-bot: I haz the power, 2015-09-02 05:35:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1321033003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1321033003/1

[commit-bot: I haz the power, 2015-09-02 06:30:25]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201617