Await promises returned from the inPageCallback

This patch teaches puppeteer to await promises returned
from the inPageCallback.

lib/Page.js

@@ -144,7 +144,7 @@ class Page extends EventEmitter {
         var name = inPageCallback.name;
         var callback = inPageCallback.callback;
         var args = await this.evaluate(callbackName => window[callbackName].__args, name);
-        var result = callback.apply(null, args);
+        var result = await Promise.resolve(callback.apply(null, args));
         await this.evaluate(assignResult, name, result);
         this._client.send('Debugger.resume');
 

test/test.js

@@ -73,6 +73,16 @@ describe('Puppeteer', function() {
             });
             expect(result).toBe(36);
         }));
+        it('should await returned promise', SX(async function() {
+            await page.setInPageCallback('callController', function(a, b) {
+                return Promise.resolve(a * b);
+            });
+
+            var result = await page.evaluate(function() {
+                return callController(3, 5);
+            });
+            expect(result).toBe(15);
+        }));
     });
 });