SetProgressBar API Implementation, fixes #635

@zcbenz , please take a look.

About the linux platform, we can support `Unity` desktop environment by using the function `unity::SetProgressFraction()` in chromium. After some tries,  we need to modify the chromium code, rename the `chromium-browser.desktop`and the env variable `CHROME_DESKTOP` to atom custom names since the progress bar in Unity relies on the them.  We can add a patch in `libchromiumcontent`, How do you think about this? 
Done.