Reuse single TrustedTypePolicy object

FIX: Don't recreated `TrustedTypePolicy` objects for every paste.

Issue ProseMirror/prosemirror#1493

src/clipboard.ts

@@ -204,13 +204,16 @@ function detachedDoc() {
   return _detachedDoc || (_detachedDoc = document.implementation.createHTMLDocument("title"))
 }
 
+let _policy: any = null
+
 function maybeWrapTrusted(html: string): string {
   let trustedTypes = (window as any).trustedTypes
   if (!trustedTypes) return html
   // With the require-trusted-types-for CSP, Chrome will block
   // innerHTML, even on a detached document. This wraps the string in
   // a way that makes the browser allow us to use its parser again.
-  return trustedTypes.createPolicy("detachedDocument", {createHTML: (s: string) => s}).createHTML(html)
+  if (!_policy) _policy = trustedTypes.createPolicy("detachedDocument", {createHTML: (s: string) => s})
+  return _policy.createHTML(html)
 }
 
 function readHTML(html: string) {