Move ash/system/session to ash/system/chromeos/session

This CL moves all files in the ash/system/session directory to
ash/system/chromeos/session and prevents their compilation on platforms
other than Chrome OS.

BUG=278247
TEST=Chrome and unit tests still compile

Review URL: https://codereview.chromium.org/196283008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256815 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256815