Drop unneeded Page Event's prefixes.

This patch drops 'Page.Event.' prefix in every puppeteer's page
event. This makes it convenient to subscribe to events by their
string value.

examples/features.js

@@ -21,7 +21,7 @@ var browser = new Browser();
 browser.newPage().then(async page => {
     var modernizrPath = path.join('..', 'third_party', 'phantomjs', 'examples', 'modernizr.js');
     await page.injectFile(modernizrPath);
-    page.on('Page.Events.ConsoleMessage', console.log);
+    page.on('ConsoleMessage', console.log);
     await page.evaluate(detectFeatures);
     browser.close();
 });

lib/Page.js

@@ -416,11 +416,11 @@ Page.ScreenshotTypes = {
 };
 
 Page.Events = {
-    ConsoleMessage: 'Page.Events.ConsoleMessage',
-    DialogOpened: 'Page.Events.DialogOpened',
-    ExceptionThrown: 'Page.Events.ExceptionThrown',
-    ResourceLoadingFailed: 'Page.Events.ResourceLoadingFailed',
-    ResponseReceived: 'Page.Events.ResponseReceived',
+    ConsoleMessage: 'ConsoleMessage',
+    DialogOpened: 'DialogOpened',
+    ExceptionThrown: 'ExceptionThrown',
+    ResourceLoadingFailed: 'ResourceLoadingFailed',
+    ResponseReceived: 'ResponseReceived',
 };
 
 module.exports = Page;

test/test.js

@@ -33,9 +33,9 @@ describe('Puppeteer', function() {
         expect(result).toBe(56);
     }));
 
-    it('Page.Events.ConsoleMessage', SX(async function() {
+    it('Page Events: ConsoleMessage', SX(async function() {
         var msgs = [];
-        page.on('Page.Events.ConsoleMessage', msg => msgs.push(msg));
+        page.on('ConsoleMessage', msg => msgs.push(msg));
         await page.evaluate(() => console.log('Message!'));
         expect(msgs).toEqual(['Message!']);
     }));