Clean up InputMethodLinuxX11 following crrev.com/48203002

http://crrev.com/48203002 makes it possible to make the implementation of InputMethodXxx family much simpler and cleaner.  Cleans InputMethodLinuxX11 following the CL.

TEST=none

Review URL: https://codereview.chromium.org/61903004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235504 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235504