For cross-browser testing and to emulate sites on other devices till now we relied on extensions (like The User-Agent Switcher for Chrome) and added command line switches to chrome to change user-agent string in Google Chrome, now you don’t need all this, since it’s built right into dev tools of Chrome now, current dev version has it.
Built-in User-Agent Switcher for Chrome
All you need to do is, open developer tools (use Ctrl +Shift+I shortcut instead) from the wrench menu>Tools>Developer Tools
Click on ‘Emulation’ panel, select Emulate Mobile and select the Model which you want to Emulate and reload the page for proper user agent spoofing and Viewport rendering.
[Thanks to François Beaufort]