[chromedriver] adding 'androidUseExistingApp' boolean flag to capabilities
    
avoids launching android process, assumes app is already started.
    
BUG=634
R=craigdh@chromium.org

Review URL: https://codereview.chromium.org/81613002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239441 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239441