Added CHROMEOS_SESSION_LOG_DIR to give incognito mode a place to save chrome session logs.
Added CHROMEOS_SESSION_LOG_DIR to give incognito mode a place to save
chrome session logs.

BUG=http://crosbug.com/6908
TEST=Ran on device in both incognito and regular mode and observed logs.

Review URL: http://codereview.chromium.org/4120012

Change-Id: I3f816d3b730461f5d3eb5910604a5f6ce10f35a9

chadversary/chromiumos.platform.login_manager,joshfire/chromiumos-platform-login_manager,chadversary/chromiumos.platform.login_manager,joshfire/chromiumos-platform-login_manager,chadversary/chromiumos.platform.login_manager,joshfire/chromiumos-platform-login_manager