TabManager: Move periodic OOM score adjustment logic to TabManagerDelegate

|update_timer_| in TabManager is only responsible for periodically
adjusting OOM scores on ChromeOS.

Since this behaviour is an implementation detail specific to ChromeOS,
the timer and associated logic should be moved to the ChromeOS
TabManagerDelegate.

Change-Id: Ic9e8f5e9925efabddc92892b8c4e7cd1e3a208ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1487872
Reviewed-by: Cheng-Yu Lee <cylee@chromium.org>
Reviewed-by: Fran├žois Doray <fdoray@chromium.org>
Commit-Queue: Fran├žois Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642903}
4 files changed