Revert "Changed width and height on youtube embed so it doesn't overflow the content area."

This reverts commit 0cb2b93416.

docs/mobile-emulation.html

@@ -31,7 +31,7 @@ mobile-specific parts on the devices you intend to support. Device emulation can
 make this process more straightforward.</p>
 
 <div class="video-container">
-  <iframe width="425" height="355" src="//www.youtube.com/embed/z7sTRdSpA04" frameborder="0" allowfullscreen=""></iframe>
+  <iframe width="640" height="360" src="//www.youtube.com/embed/z7sTRdSpA04" frameborder="0" allowfullscreen=""></iframe>
 </div>
 
 <p>DevTools contains built in presets for a number of devices, this allows you to emulate and debug mobile viewport issues like CSS media query breakpoints. Selecting a device preset automatically enables a number of settings:</p>