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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

Review URL: https://codereview.chromium.org/1321033003

git-svn-id: svn://svn.chromium.org/blink/trunk@201617 bbb929c8-8fbe-4397-9dbb-9b2b20218538