Sync the URL constructor with the spec

https://url.spec.whatwg.org/#api

This changes the generated code, but ought not be observable, as any
call to `new URL(x, urlObject)` will now instead behaves as
`new URL(x, urlObject.toString())`.

BUG=460722

Review URL: https://codereview.chromium.org/1464133002

Cr-Commit-Position: refs/heads/master@{#361342}

third_party/WebKit/Source/core/dom/DOMURL.h

@@ -52,11 +52,6 @@ public:
     {
         return new DOMURL(url, KURL(KURL(), base), exceptionState);
     }
-    static DOMURL* create(const String& url, DOMURL* base, ExceptionState& exceptionState)
-    {
-        ASSERT(base);
-        return new DOMURL(url, base->m_url, exceptionState);
-    }
 
     static String createObjectURL(ExecutionContext*, Blob*, ExceptionState&);
     static void revokeObjectURL(ExecutionContext*, const String&);

third_party/WebKit/Source/core/dom/URL.idl

@@ -27,11 +27,7 @@
 // https://url.spec.whatwg.org/#url
 
 [
-    // TODO(philipj): There should only be one constructor:
-    // Constructor(USVString url, optional USVString base)
-    Constructor(USVString url),
-    Constructor(USVString url, USVString base),
-    Constructor(USVString url, URL base),
+    Constructor(USVString url, optional USVString base),
     Exposed=(Window,Worker),
     ImplementedAs=DOMURL,
     RaisesException=Constructor,