Fold MojoViewport C++ class into NativeViewportAndroid

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

Review URL: https://codereview.chromium.org/99683003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238905 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238905