fix(page): fix race condition in WaitTask (#2739)

This patch eliminates a common race condition with WaitTask, that
happens when predicate function gets resolved right before the execution
context gets destroyed.
This situation results in a "Cannot find context with specified id undefined"
exception.

Credits go to @jakub300 for his wonderful [investigation](#1325 (comment)).

Fixes #1325.

lib/FrameManager.js

@@ -869,7 +869,9 @@ class WaitTask {
     }
 
     // Ignore timeouts in pageScript - we track timeouts ourselves.
-    if (!error && await this._frame.evaluate(s => !s, success)) {
+    // If the frame's execution context has already changed, `frame.evaluate` will
+    // throw an error - ignore this predicate run altogether.
+    if (!error && await this._frame.evaluate(s => !s, success).catch(e => true)) {
       await success.dispose();
       return;
     }

test/frame.spec.js

@@ -84,6 +84,14 @@ module.exports.addTests = function({testRunner, expect}) {
       await page.evaluate(() => window.__FOO = 1);
       await watchdog;
     });
+    it('should work when resolved right before execution context disposal', async({page, server}) => {
+      await page.evaluateOnNewDocument(() => window.__RELOADED = true);
+      await page.waitForFunction(() => {
+        if (!window.__RELOADED)
+          window.location.reload();
+        return true;
+      });
+    });
     it('should poll on interval', async({page, server}) => {
       let success = false;
       const startTime = Date.now();