Adding a LaunchApp command to Chromedriver using the Chrome management API in the background extension, this will give developers an improved way to launch apps over having to navigate to the chrome://apps page and click on the app tile.

BUG=

Review URL: https://codereview.chromium.org/114403005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245167 0039d316-1c4b-4281-b951-d872f2087c98