Form-associated custom elements: Fix the initial value of 'submission value'

According to https://html.spec.whatwg.org/C/#face-submission-value, the
initial value should be null, not an empty string.

Bug: 1051292
Bug: https://github.com/whatwg/html/issues/5263
Change-Id: Ifa92aaca47bfce1e51aa0fd3dc9ebb9ab9b15d91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050148Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741277}

third_party/blink/renderer/core/html/custom/element_internals.cc

@@ -47,7 +47,6 @@ class CustomStatesTokenList : public DOMTokenList {
 };
 
 ElementInternals::ElementInternals(HTMLElement& target) : target_(target) {
-  value_.SetUSVString(String());
 }
 
 void ElementInternals::Trace(Visitor* visitor) {

third_party/blink/web_tests/external/wpt/custom-elements/form-associated/ElementInternals-setFormValue.html

@@ -77,7 +77,7 @@ promise_test(t => {
   $('[name=name-usv]').value = USV_INPUT;
   $('[name=name-file]').value = new File(['file content'], FILE_NAME);
   return submitPromise(t).then(query => {
-    assert_equals(query, `?name-pd1=value-pd1&name-ce1=&name-usv=${encodeURIComponent(USV_OUTPUT)}&name-file=${FILE_NAME}`);
+    assert_equals(query, `?name-pd1=value-pd1&name-usv=${encodeURIComponent(USV_OUTPUT)}&name-file=${FILE_NAME}`);
   });
 }, 'Single value - Non-empty name exists');
 
@@ -90,7 +90,7 @@ promise_test(t => {
       '<iframe name="if1"></iframe>';
   $('my-control').value = null;
   return submitPromise(t).then(query => {
-    assert_equals(query, '?name-pd1=value-pd1&name-ce2=');
+    assert_equals(query, '?name-pd1=value-pd1');
   });
 }, 'Null value should submit nothing');