Make page.url() return URL synchronously.

page.url() is just a shortcut to page.mainFrame().url().

References #39.

docs/api.md

@@ -353,7 +353,9 @@ In case of multiple pages in one browser, each page can have its own viewport si
 - returns: <[Promise]> Promise which resolves when the value is set.
 
 #### page.url()
-- returns: <[Promise]<[string]>> Promise which resolves with the current page url.
+- returns: <[string]> Current page url.
+
+This is a shortcut for [page.mainFrame().url()](#frameurl)
 
 #### page.userAgent()
 - returns: <[string]> Returns user agent.

lib/Page.js

@@ -236,10 +236,10 @@ class Page extends EventEmitter {
   }
 
   /**
-   * @return {!Promise<string>}
+   * @return {!string}
    */
-  async url() {
-    return this.evaluate(() => window.location.href);
+  url() {
+    return this.mainFrame().url();
   }
 
   /**

test/test.js

@@ -778,9 +778,9 @@ describe('Puppeteer', function() {
 
   describe('Page.url', function() {
     it('should work', SX(async function() {
-      expect(await page.url()).toBe('about:blank');
+      expect(page.url()).toBe('about:blank');
       await page.navigate(EMPTY_PAGE);
-      expect(await page.url()).toBe(EMPTY_PAGE);
+      expect(page.url()).toBe(EMPTY_PAGE);
     }));
   });