feat(platform-server): add API to render Module and ModuleFactory to string (#14381)

- PlatformState provides an interface to serialize the current Platform State as a string or Document.

- renderModule and renderModuleFactory are convenience methods to wait for Angular Application to stabilize and then render the state to a string.

- refactor code to remove defaultDoc from DomAdapter and inject DOCUMENT where it's needed.

modules/@angular/platform-browser/test/browser/meta_spec.ts

@@ -14,9 +14,8 @@ import {expect} from '@angular/platform-browser/testing/matchers';
 
 export function main() {
   describe('Meta service', () => {
-
-    const metaService: Meta = new Meta(getDOM());
-    const doc: HTMLDocument = getDOM().defaultDoc();
+    const doc: HTMLDocument = getDOM().createHtmlDocument();
+    const metaService: Meta = new Meta(doc);
     let defaultMeta: HTMLMetaElement;
 
     beforeEach(() => {