Make CSSToggleMap.set("none", ...) throw SyntaxError.

This implements the one change in the spec edit at
tabatkins/css-toggle@c034d41
that we didn't already implement.  (The spec edit was fixing an issue
that I filed, and I'd already implemented the other changes in the spec
edit, but hadn't thought about this case.)

(Support for toggles is behind the CSSToggles flag, which is currently
off.)

Bug: 1250716
Change-Id: Ic205ee56cd2fac9508688f3e58ecfc5db2d45268
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3859688
Reviewed-by: Joey Arhar <[email protected]>
Commit-Queue: David Baron <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1039926}

css/css-toggle/toggle-api.tentative.html

@@ -79,6 +79,12 @@
   assert_equals(b.toggles.get("newtoggle"), undefined, 'b.toggles.get("newtoggle") after rename');
   assert_equals(b.toggles.get("newname"), t, 'b.toggles.get("newname") after rename');
 
+  assert_throws_dom("SyntaxError", () => { a.toggles.set("none", t); },
+                    "setting toggle_name to 'none'");
+  assert_equals(a.toggles.size, 0, "a.toggles.size after failed set");
+  assert_equals(b.toggles.size, 1, "b.toggles.size after failed set");
+  assert_equals(b.toggles.get("newname"), t, "b.toggles.get after failed set");
+
   // TODO(https://crbug.com/1250716): Should the toggle on a be
   // re-created at some point?  If so, when?