Introduce ShowState and WMEvent
* DefaultShowState will be used as a current implementation.
 Later, I'll add a flag to switch new impl.
* Move maximized, axis maximized logic to DefaultShowState class.

This is one necessary step to introduce full state machine.

BUG=318325
R=pkotwicz@chromium.org

Review URL: https://codereview.chromium.org/148473007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248785 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248785