GitLab

Model variables and logic were stored in TraySessionLengthLimit. This CL
adds SessionLengthLimitModel to SystemTrayModel and separates them.

In the upcoming CL, session length limit notification will be generated
outside TraySessionLengthLimit, as it will be removed with
UnifiedSystemTray launch.

TEST=TraySessionLengthLimitTest
BUG=none

Change-Id: Ic3994f45f0ad48460654512fa8f8e23874503a73
Reviewed-on: https://chromium-review.googlesource.com/1015444Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551534}