Have (new URLSearchParams(initString)) skip initial '?'.

third_party/WebKit/LayoutTests/fast/domurl/urlsearchparams-constructor.html

Index: third_party/WebKit/LayoutTests/fast/domurl/urlsearchparams-constructor.html
diff --git a/third_party/WebKit/LayoutTests/fast/domurl/urlsearchparams-constructor.html b/third_party/WebKit/LayoutTests/fast/domurl/urlsearchparams-constructor.html
index 33a446e32332b7ce0cdd8c79d57ed0ff645dfe3b..16cb9e7dfac5d651d01cf93e808f8b5052d179cd 100644
--- a/third_party/WebKit/LayoutTests/fast/domurl/urlsearchparams-constructor.html
+++ b/third_party/WebKit/LayoutTests/fast/domurl/urlsearchparams-constructor.html
@@ -33,6 +33,20 @@ test(function() {
     assert_equals(params + '', 'a=b');
     params = new URLSearchParams(params);
     assert_equals(params + '', 'a=b');
+
+    // Leading '?' should be ignored.
+    params = new URLSearchParams('?a=b');
+    assert_equals(params + '', 'a=b');
+
+    // https://url.spec.whatwg.org/#concept-urlencoded-byte-serializer
+    // is performed upon stringification of |params|, requiring that
+    // '?' is percent encoded.
+    params = new URLSearchParams('??a=b');
+    assert_equals(params + '', '%3Fa=b');
+    params = new URLSearchParams('?');
+    assert_equals(params + '', '');
+    params = new URLSearchParams('??');
+    assert_equals(params + '', '%3F=');
 }, 'Basic URLSearchParams construction');
 
 test(function() {