Remove separate thread for the shell on Android.

The shell is now run on the main (UI) thread along with the NativeViewport stuff.

R=abarth@chromium.org
http://crbug.com/324637

Review URL: https://codereview.chromium.org/100623003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238474 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238474