fix(page): fix repetitative setContent calls (#3666)

Fix #3665.

lib/FrameManager.js

@@ -1201,6 +1201,7 @@ class LifecycleWatcher {
     this._terminationPromise = new Promise(fulfill => {
       this._terminationCallback = fulfill;
     });
+    this._checkLifecycleComplete();
   }
 
   /**

test/page.spec.js

@@ -601,6 +601,10 @@ module.exports.addTests = function({testRunner, expect, headless}) {
       imgResponse.end();
       await contentPromise;
     });
+    it('should work fast enough', async({page, server}) => {
+      for (let i = 0; i < 20; ++i)
+        await page.setContent('<div>yo</div>');
+    });
   });
 
   describe('Page.setBypassCSP', function() {